| pkg-config --libs libffi
=> "dyld: Symbol not found: _iconv\n"
=> "  Referenced from: /usr/lib/libcups.2.dylib\n"
=> "  Expected in: /opt/local/lib/libiconv.2.dylib\n"
=> " in /usr/lib/libcups.2.dylib\n"
"/usr/bin/clang -o conftest -I/opt/local/include/ruby-2.4.0/x86_64-darwin15 -I/opt/local/include/ruby-2.4.0/ruby/backward -I/opt/local/include/ruby-2.4.0 -I. -I/opt/local/include -I/opt/local/include -D_XOPEN_SOURCE -D_DARWIN_C_SOURCE -D_DARWIN_UNLIMITED_SELECT -D_REENTRANT   -pipe -Os -fno-common conftest.c  -L. -L/opt/local/lib -L/opt/local/lib -L. -L/opt/local/lib -Wl,-headerpad_max_install_names -fstack-protector -L/usr/local/lib -L/opt/local/lib   -arch x86_64   -lruby.2.4.0  -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/clang -o conftest -I/opt/local/include/ruby-2.4.0/x86_64-darwin15 -I/opt/local/include/ruby-2.4.0/ruby/backward -I/opt/local/include/ruby-2.4.0 -I. -I/opt/local/include -I/opt/local/include -D_XOPEN_SOURCE -D_DARWIN_C_SOURCE -D_DARWIN_UNLIMITED_SELECT -D_REENTRANT   -pipe -Os -fno-common conftest.c  -L. -L/opt/local/lib -L/opt/local/lib -L. -L/opt/local/lib -Wl,-headerpad_max_install_names -fstack-protector -L/usr/local/lib -L/opt/local/lib   -arch x86_64   -lruby.2.4.0  -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 */

| pkg-config --cflags-only-I libffi
=> "dyld: Symbol not found: _iconv\n"
=> "  Referenced from: /usr/lib/libcups.2.dylib\n"
=> "  Expected in: /opt/local/lib/libiconv.2.dylib\n"
=> " in /usr/lib/libcups.2.dylib\n"
| pkg-config --cflags libffi
=> "dyld: Symbol not found: _iconv\n"
=> "  Referenced from: /usr/lib/libcups.2.dylib\n"
=> "  Expected in: /opt/local/lib/libiconv.2.dylib\n"
=> " in /usr/lib/libcups.2.dylib\n"
| pkg-config --libs-only-l libffi
=> "dyld: Symbol not found: _iconv\n"
=> "  Referenced from: /usr/lib/libcups.2.dylib\n"
=> "  Expected in: /opt/local/lib/libiconv.2.dylib\n"
=> " in /usr/lib/libcups.2.dylib\n"