"pkg-config --exists openssl"
| pkg-config --libs openssl
=> "-lssl -lcrypto \n"
"gcc -o conftest -I../../../.ext/include/x86_64-linux -I../../.././include -I../../.././ext/digest/rmd160     -O3 -fno-fast-math -ggdb3 -Wall -Wextra -Wno-unused-parameter -Wno-parentheses -Wno-long-long -Wno-missing-field-initializers -Wunused-variable -Wpointer-arith -Wwrite-strings -Wdeclaration-after-statement -Wimplicit-function-declaration -Wdeprecated-declarations -Wno-packed-bitfield-compat  -fPIC conftest.c  -L. -L../../.. -L. -fstack-protector -rdynamic -Wl,-export-dynamic     -Wl,-rpath,/usr/local/rvm/rubies/ruby-2.3.8/lib -L/usr/local/rvm/rubies/ruby-2.3.8/lib -lruby-static -lssl -lcrypto -lpthread -ldl -lcrypt -lm   -lc"
checked program was:
/* begin */
1: #include "ruby.h"
2: 
3: int main(int argc, char **argv)
4: {
5:   return 0;
6: }
/* end */

| pkg-config --cflags-only-I openssl
=> "\n"
| pkg-config --cflags-only-other openssl
=> "\n"
| pkg-config --libs-only-l openssl
=> "-lssl -lcrypto \n"
package configuration for openssl
cflags: 
ldflags: 
libs: -lssl -lcrypto

have_library: checking for main() in -lcrypto... -------------------- yes

"gcc -o conftest -I../../../.ext/include/x86_64-linux -I../../.././include -I../../.././ext/digest/rmd160      -O3 -fno-fast-math -ggdb3 -Wall -Wextra -Wno-unused-parameter -Wno-parentheses -Wno-long-long -Wno-missing-field-initializers -Wunused-variable -Wpointer-arith -Wwrite-strings -Wdeclaration-after-statement -Wimplicit-function-declaration -Wdeprecated-declarations -Wno-packed-bitfield-compat  -fPIC  conftest.c  -L. -L../../.. -L. -fstack-protector -rdynamic -Wl,-export-dynamic      -lssl -lcrypto -Wl,-rpath,/usr/local/rvm/rubies/ruby-2.3.8/lib -L/usr/local/rvm/rubies/ruby-2.3.8/lib -lruby-static -lcrypto  -lssl -lcrypto -lpthread -ldl -lcrypt -lm   -lc"
conftest.c: In function ‘t’:
conftest.c:13:43: warning: cast between incompatible function types from ‘int (*)(int,  char **)’ to ‘void (*)()’ [-Wcast-function-type]
 int t(void) { void ((*volatile p)()); p = (void ((*)()))main; return !p; }
                                           ^
checked program was:
/* begin */
 1: #include "ruby.h"
 2: 
 3: /*top*/
 4: extern int t(void);
 5: int main(int argc, char **argv)
 6: {
 7:   if (argc > 1000000) {
 8:     printf("%p", &t);
 9:   }
10: 
11:   return 0;
12: }
13: int t(void) { void ((*volatile p)()); p = (void ((*)()))main; return !p; }
/* end */

--------------------

have_func: checking for RIPEMD160_Transform() in openssl/ripemd.h with -Werror=deprecated-declarations... -------------------- yes

"gcc -o conftest -I../../../.ext/include/x86_64-linux -I../../.././include -I../../.././ext/digest/rmd160      -O3 -fno-fast-math -ggdb3 -Wall -Wextra -Wno-unused-parameter -Wno-parentheses -Wno-long-long -Wno-missing-field-initializers -Wunused-variable -Wpointer-arith -Wwrite-strings -Wdeclaration-after-statement -Wimplicit-function-declaration -Wdeprecated-declarations -Wno-packed-bitfield-compat  -fPIC  conftest.c  -L. -L../../.. -L. -fstack-protector -rdynamic -Wl,-export-dynamic     -lcrypto  -lssl -lcrypto -Wl,-rpath,/usr/local/rvm/rubies/ruby-2.3.8/lib -L/usr/local/rvm/rubies/ruby-2.3.8/lib -lruby-static -Werror=deprecated-declarations -lcrypto  -lssl -lcrypto -lpthread -ldl -lcrypt -lm   -lc"
checked program was:
/* begin */
 1: #include "ruby.h"
 2: 
 3: #include <openssl/ripemd.h>
 4: 
 5: /*top*/
 6: extern int t(void);
 7: int main(int argc, char **argv)
 8: {
 9:   if (argc > 1000000) {
10:     printf("%p", &t);
11:   }
12: 
13:   return 0;
14: }
15: int t(void) { void ((*volatile p)()); p = (void ((*)()))RIPEMD160_Transform; return !p; }
/* end */

--------------------

have_header: checking for openssl/ripemd.h... -------------------- yes

"gcc -E -I../../../.ext/include/x86_64-linux -I../../.././include -I../../.././ext/digest/rmd160      -O3 -fno-fast-math -ggdb3 -Wall -Wextra -Wno-unused-parameter -Wno-parentheses -Wno-long-long -Wno-missing-field-initializers -Wunused-variable -Wpointer-arith -Wwrite-strings -Wdeclaration-after-statement -Wimplicit-function-declaration -Wdeprecated-declarations -Wno-packed-bitfield-compat  -fPIC  -Werror=deprecated-declarations conftest.c -o conftest.i"
checked program was:
/* begin */
1: #include "ruby.h"
2: 
3: #include <openssl/ripemd.h>
/* end */

--------------------

have_type: checking for RIPEMD160_CTX in openssl/ripemd.h... -------------------- yes

"gcc -I../../../.ext/include/x86_64-linux -I../../.././include -I../../.././ext/digest/rmd160      -O3 -fno-fast-math -ggdb3 -Wall -Wextra -Wno-unused-parameter -Wno-parentheses -Wno-long-long -Wno-missing-field-initializers -Wunused-variable -Wpointer-arith -Wwrite-strings -Wdeclaration-after-statement -Wimplicit-function-declaration -Wdeprecated-declarations -Wno-packed-bitfield-compat  -fPIC    -c conftest.c"
checked program was:
/* begin */
1: #include "ruby.h"
2: 
3: #include <openssl/ripemd.h>
4: 
5: /*top*/
6: typedef RIPEMD160_CTX conftest_type;
7: int conftestval[sizeof(conftest_type)?1:-1];
/* end */

--------------------

have_header: checking for sys/cdefs.h... -------------------- yes

"gcc -E -I../../../.ext/include/x86_64-linux -I../../.././include -I../../.././ext/digest/rmd160      -O3 -fno-fast-math -ggdb3 -Wall -Wextra -Wno-unused-parameter -Wno-parentheses -Wno-long-long -Wno-missing-field-initializers -Wunused-variable -Wpointer-arith -Wwrite-strings -Wdeclaration-after-statement -Wimplicit-function-declaration -Wdeprecated-declarations -Wno-packed-bitfield-compat  -fPIC   conftest.c -o conftest.i"
checked program was:
/* begin */
1: #include "ruby.h"
2: 
3: #include <sys/cdefs.h>
/* end */

--------------------

extconf.h is:
/* begin */
 1: #ifndef EXTCONF_H
 2: #define EXTCONF_H
 3: #define NDEBUG 1
 4: #define HAVE_CONFIG_H 1
 5: #define HAVE_RIPEMD160_TRANSFORM 1
 6: #define HAVE_OPENSSL_RIPEMD_H 1
 7: #define HAVE_TYPE_RIPEMD160_CTX 1
 8: #define RMD160_USE_OPENSSL 1
 9: #define HAVE_SYS_CDEFS_H 1
10: #endif
/* end */