gil_release_version.h is:
/* begin */
1: #ifndef GIL_RELEASE_VERSION_H
2: #define GIL_RELEASE_VERSION_H
3: #endif
/* end */

unicode_support_version.h is:
/* begin */
1: #ifndef UNICODE_SUPPORT_VERSION_H
2: #define UNICODE_SUPPORT_VERSION_H
3: #endif
/* end */

have_library: checking for SQLConnect() in -ldb2cli... -------------------- yes

"i686-w64-mingw32-gcc -o conftest.exe -IC:/Ruby25/include/ruby-2.5.0/i386-mingw32 -IC:/Ruby25/include/ruby-2.5.0/ruby/backward -IC:/Ruby25/include/ruby-2.5.0 -I. -IC:\clidriver\32-bit\clidriver/include -D__USE_MINGW_ANSI_STDIO=1 -DFD_SETSIZE=2048 -D_WIN32_WINNT=0x0501 -D__MINGW_USE_VC2005_COMPAT -D_FILE_OFFSET_BITS=64  -O3 -fno-fast-math -fstack-protector-strong conftest.c  -L. -LC:/Ruby25/lib -LC:\clidriver\32-bit\clidriver/lib -L. -pipe -s -fstack-protector-strong     -lmsvcrt-ruby250  -lgmp -lshell32 -lws2_32 -liphlpapi -limagehlp -lshlwapi  "
checked program was:
/* begin */
1: #include "ruby.h"
2: 
3: #include <winsock2.h>
4: #include <windows.h>
5: int main(int argc, char **argv)
6: {
7:   return 0;
8: }
/* end */

"i686-w64-mingw32-gcc -o conftest.exe -IC:/Ruby25/include/ruby-2.5.0/i386-mingw32 -IC:/Ruby25/include/ruby-2.5.0/ruby/backward -IC:/Ruby25/include/ruby-2.5.0 -I. -IC:\clidriver\32-bit\clidriver/include -D__USE_MINGW_ANSI_STDIO=1 -DFD_SETSIZE=2048 -D_WIN32_WINNT=0x0501 -D__MINGW_USE_VC2005_COMPAT -D_FILE_OFFSET_BITS=64  -O3 -fno-fast-math -fstack-protector-strong conftest.c  -L. -LC:/Ruby25/lib -LC:\clidriver\32-bit\clidriver/lib -L. -pipe -s -fstack-protector-strong     -lmsvcrt-ruby250 -ldb2cli  -lgmp -lshell32 -lws2_32 -liphlpapi -limagehlp -lshlwapi  "
conftest.c: In function 't':
conftest.c:16:57: error: 'SQLConnect' undeclared (first use in this function); did you mean 'WSAConnect'?
   16 | int t(void) { void ((*volatile p)()); p = (void ((*)()))SQLConnect; return !p; }
      |                                                         ^~~~~~~~~~
      |                                                         WSAConnect
conftest.c:16:57: note: each undeclared identifier is reported only once for each function it appears in
checked program was:
/* begin */
 1: #include "ruby.h"
 2: 
 3: #include <winsock2.h>
 4: #include <windows.h>
 5: 
 6: /*top*/
 7: extern int t(void);
 8: int main(int argc, char **argv)
 9: {
10:   if (argc > 1000000) {
11:     printf("%p", &t);
12:   }
13: 
14:   return 0;
15: }
16: int t(void) { void ((*volatile p)()); p = (void ((*)()))SQLConnect; return !p; }
/* end */

"i686-w64-mingw32-gcc -o conftest.exe -IC:/Ruby25/include/ruby-2.5.0/i386-mingw32 -IC:/Ruby25/include/ruby-2.5.0/ruby/backward -IC:/Ruby25/include/ruby-2.5.0 -I. -IC:\clidriver\32-bit\clidriver/include -D__USE_MINGW_ANSI_STDIO=1 -DFD_SETSIZE=2048 -D_WIN32_WINNT=0x0501 -D__MINGW_USE_VC2005_COMPAT -D_FILE_OFFSET_BITS=64  -O3 -fno-fast-math -fstack-protector-strong conftest.c  -L. -LC:/Ruby25/lib -LC:\clidriver\32-bit\clidriver/lib -L. -pipe -s -fstack-protector-strong     -lmsvcrt-ruby250 -ldb2cli  -lgmp -lshell32 -lws2_32 -liphlpapi -limagehlp -lshlwapi  "
checked program was:
/* begin */
 1: #include "ruby.h"
 2: 
 3: #include <winsock2.h>
 4: #include <windows.h>
 5: 
 6: /*top*/
 7: extern int t(void);
 8: int main(int argc, char **argv)
 9: {
10:   if (argc > 1000000) {
11:     printf("%p", &t);
12:   }
13: 
14:   return 0;
15: }
16: extern void SQLConnect();
17: int t(void) { SQLConnect(); return 0; }
/* end */

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

have_header: checking for gil_release_version.h... -------------------- yes

"i686-w64-mingw32-gcc -E -IC:/Ruby25/include/ruby-2.5.0/i386-mingw32 -IC:/Ruby25/include/ruby-2.5.0/ruby/backward -IC:/Ruby25/include/ruby-2.5.0 -I. -IC:\clidriver\32-bit\clidriver/include -D__USE_MINGW_ANSI_STDIO=1 -DFD_SETSIZE=2048 -D_WIN32_WINNT=0x0501 -D__MINGW_USE_VC2005_COMPAT -D_FILE_OFFSET_BITS=64  -O3 -fno-fast-math -fstack-protector-strong  conftest.c -o conftest.i"
checked program was:
/* begin */
1: #include "ruby.h"
2: 
3: #include <winsock2.h>
4: #include <windows.h>
5: #include <gil_release_version.h>
/* end */

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

have_header: checking for unicode_support_version.h... -------------------- yes

"i686-w64-mingw32-gcc -E -IC:/Ruby25/include/ruby-2.5.0/i386-mingw32 -IC:/Ruby25/include/ruby-2.5.0/ruby/backward -IC:/Ruby25/include/ruby-2.5.0 -I. -IC:\clidriver\32-bit\clidriver/include -D__USE_MINGW_ANSI_STDIO=1 -DFD_SETSIZE=2048 -D_WIN32_WINNT=0x0501 -D__MINGW_USE_VC2005_COMPAT -D_FILE_OFFSET_BITS=64  -O3 -fno-fast-math -fstack-protector-strong  conftest.c -o conftest.i"
checked program was:
/* begin */
1: #include "ruby.h"
2: 
3: #include <winsock2.h>
4: #include <windows.h>
5: #include <unicode_support_version.h>
/* end */

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