"gcc -I../../../.ext/include/x86_64-linux -I../../.././include -I../../.././ext/io/wait    -O3 -ggdb3 -Wall -Wextra -Wdeprecated-declarations -Wduplicated-cond -Wimplicit-function-declaration -Wimplicit-int -Wmisleading-indentation -Wpointer-arith -Wwrite-strings -Wimplicit-fallthrough=0 -Wmissing-noreturn -Wno-cast-function-type -Wno-constant-logical-operand -Wno-long-long -Wno-missing-field-initializers -Wno-overlength-strings -Wno-packed-bitfield-compat -Wno-parentheses-equality -Wno-self-assign -Wno-tautological-compare -Wno-unused-parameter -Wno-unused-value -Wsuggest-attribute=format -Wsuggest-attribute=noreturn -Wunused-variable  -fPIC   -c conftest.c"
conftest.c:6:3: error: #error 
 # error
   ^~~~~
conftest.c:7:1: error: expected identifier or ‘(’ before ‘|’ token
 |:/ === DOSISH undefined === /:|
 ^
cc1: warning: unrecognized command line option ‘-Wno-self-assign’
cc1: warning: unrecognized command line option ‘-Wno-parentheses-equality’
cc1: warning: unrecognized command line option ‘-Wno-constant-logical-operand’
checked program was:
/* begin */
1: #include "ruby.h"
2: 
3: #include <ruby.h>
4: /*top*/
5: #ifndef DOSISH
6: # error
7: |:/ === DOSISH undefined === /:|
8: #endif
/* end */

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

"gcc -E -I../../../.ext/include/x86_64-linux -I../../.././include -I../../.././ext/io/wait    -O3 -ggdb3 -Wall -Wextra -Wdeprecated-declarations -Wduplicated-cond -Wimplicit-function-declaration -Wimplicit-int -Wmisleading-indentation -Wpointer-arith -Wwrite-strings -Wimplicit-fallthrough=0 -Wmissing-noreturn -Wno-cast-function-type -Wno-constant-logical-operand -Wno-long-long -Wno-missing-field-initializers -Wno-overlength-strings -Wno-packed-bitfield-compat -Wno-parentheses-equality -Wno-self-assign -Wno-tautological-compare -Wno-unused-parameter -Wno-unused-value -Wsuggest-attribute=format -Wsuggest-attribute=noreturn -Wunused-variable  -fPIC  conftest.c -o conftest.i"
checked program was:
/* begin */
1: #include "ruby.h"
2: 
3: #include <sys/ioctl.h>
/* end */

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

have_macro: checking for FIONREAD in sys/ioctl.h,sys/ioctl.h... -------------------- yes

"gcc -I../../../.ext/include/x86_64-linux -I../../.././include -I../../.././ext/io/wait    -O3 -ggdb3 -Wall -Wextra -Wdeprecated-declarations -Wduplicated-cond -Wimplicit-function-declaration -Wimplicit-int -Wmisleading-indentation -Wpointer-arith -Wwrite-strings -Wimplicit-fallthrough=0 -Wmissing-noreturn -Wno-cast-function-type -Wno-constant-logical-operand -Wno-long-long -Wno-missing-field-initializers -Wno-overlength-strings -Wno-packed-bitfield-compat -Wno-parentheses-equality -Wno-self-assign -Wno-tautological-compare -Wno-unused-parameter -Wno-unused-value -Wsuggest-attribute=format -Wsuggest-attribute=noreturn -Wunused-variable  -fPIC   -c conftest.c"
checked program was:
/* begin */
1: #include "ruby.h"
2: 
3: #include <sys/ioctl.h>
4: #include <sys/ioctl.h>
5: /*top*/
6: #ifndef FIONREAD
7: # error
8: |:/ === FIONREAD undefined === /:|
9: #endif
/* end */

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

extconf.h is:
/* begin */
1: #ifndef EXTCONF_H
2: #define EXTCONF_H
3: #define HAVE_SYS_IOCTL_H 1
4: #define FIONREAD_HEADER <sys/ioctl.h>
5: #endif
/* end */