cat mkmf.log

mkmf.log

have_header: checking for mpg123.h... -------------------- yes

"/usr/bin/cc -o conftest -I/opt/boxen/rbenv/versions/2.0.0-p247/include/ruby-2.0.0/x86_64-darwin12.4.0 -I/opt/boxen/rbenv/versions/2.0.0-p247/include/ruby-2.0.0/ruby/backward -I/opt/boxen/rbenv/versions/2.0.0-p247/include/ruby-2.0.0 -I. -I'/opt/boxen/rbenv/versions/2.0.0-p247/include'  -D_XOPEN_SOURCE -D_DARWIN_C_SOURCE -D_DARWIN_UNLIMITED_SELECT -D_REENTRANT   -I/opt/X11/include -O3 -Wno-error=shorten-64-to-32  -pipe conftest.c  -L. -L/opt/boxen/rbenv/versions/2.0.0-p247/lib -L. -L'/opt/boxen/rbenv/versions/2.0.0-p247/lib'  -L/usr/local/lib     -lruby-static  -lpthread -ldl -lobjc "
checked program was:
/* begin */
1: #include "ruby.h"
2:
3: int main(int argc, char **argv)
4: {
5:   return 0;
6: }
/* end */

"/usr/bin/cc -E -I/opt/boxen/rbenv/versions/2.0.0-p247/include/ruby-2.0.0/x86_64-darwin12.4.0 -I/opt/boxen/rbenv/versions/2.0.0-p247/include/ruby-2.0.0/ruby/backward -I/opt/boxen/rbenv/versions/2.0.0-p247/include/ruby-2.0.0 -I. -I'/opt/boxen/rbenv/versions/2.0.0-p247/include'  -D_XOPEN_SOURCE -D_DARWIN_C_SOURCE -D_DARWIN_UNLIMITED_SELECT -D_REENTRANT   -I/opt/X11/include -O3 -Wno-error=shorten-64-to-32  -pipe  conftest.c -o conftest.i"
checked program was:
/* begin */
1: #include "ruby.h"
2:
3: #include <mpg123.h>
/* end */

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

have_library: checking for main() in -lmpg123... -------------------- no

"/usr/bin/cc -o conftest -I/opt/boxen/rbenv/versions/2.0.0-p247/include/ruby-2.0.0/x86_64-darwin12.4.0 -I/opt/boxen/rbenv/versions/2.0.0-p247/include/ruby-2.0.0/ruby/backward -I/opt/boxen/rbenv/versions/2.0.0-p247/include/ruby-2.0.0 -I. -I'/opt/boxen/rbenv/versions/2.0.0-p247/include'  -D_XOPEN_SOURCE -D_DARWIN_C_SOURCE -D_DARWIN_UNLIMITED_SELECT -D_REENTRANT   -I/opt/X11/include -O3 -Wno-error=shorten-64-to-32  -pipe conftest.c  -L. -L/opt/boxen/rbenv/versions/2.0.0-p247/lib -L. -L'/opt/boxen/rbenv/versions/2.0.0-p247/lib'  -L/usr/local/lib     -lruby-static -lmpg123  -lpthread -ldl -lobjc "
conftest.c:5:57: error: use of undeclared identifier 'main'
int t(void) { void ((*volatile p)()); p = (void ((*)()))main; return 0; }
                                                        ^
1 error generated.
checked program was:
/* begin */
 1: #include "ruby.h"
 2:
 3: /*top*/
 4: extern int t(void);
 5: int t(void) { void ((*volatile p)()); p = (void ((*)()))main; return 0; }
 6: int main(int argc, char **argv)
 7: {
 8:   if (argc > 1000000) {
 9:     printf("%p", &t);
10:   }
11:
12:   return 0;
13: }
/* end */

"/usr/bin/cc -o conftest -I/opt/boxen/rbenv/versions/2.0.0-p247/include/ruby-2.0.0/x86_64-darwin12.4.0 -I/opt/boxen/rbenv/versions/2.0.0-p247/include/ruby-2.0.0/ruby/backward -I/opt/boxen/rbenv/versions/2.0.0-p247/include/ruby-2.0.0 -I. -I'/opt/boxen/rbenv/versions/2.0.0-p247/include'  -D_XOPEN_SOURCE -D_DARWIN_C_SOURCE -D_DARWIN_UNLIMITED_SELECT -D_REENTRANT   -I/opt/X11/include -O3 -Wno-error=shorten-64-to-32  -pipe conftest.c  -L. -L/opt/boxen/rbenv/versions/2.0.0-p247/lib -L. -L'/opt/boxen/rbenv/versions/2.0.0-p247/lib'  -L/usr/local/lib     -lruby-static -lmpg123  -lpthread -ldl -lobjc "
conftest.c:5:15: warning: implicit declaration of function 'main' is invalid in C99 [-Wimplicit-function-declaration]
int t(void) { main(); return 0; }
              ^
1 warning generated.
ld: library not found for -lmpg123
clang: error: linker command failed with exit code 1 (use -v to see invocation)
checked program was:
/* begin */
 1: #include "ruby.h"
 2:
 3: /*top*/
 4: extern int t(void);
 5: int t(void) { main(); return 0; }
 6: int main(int argc, char **argv)
 7: {
 8:   if (argc > 1000000) {
 9:     printf("%p", &t);
10:   }
11:
12:   return 0;
13: }
/* end */

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

I'm having a similar issue as this. Were you able to resolve it?

I ended up formatting my machine a year later and it suddently worked, sorry!