"pkg-config --exists libffi"
| pkg-config --libs libffi
=> "-L/usr/lib64/../lib64 -lffi\n"
"x86_64-pc-linux-gnu-gcc -o conftest -I/usr/include/ruby-2.5.0/x86_64-linux -I/usr/include/ruby-2.5.0/ruby/backward -I/usr/include/ruby-2.5.0 -I.    -march=native -O2 -pipe -fno-lto -fno-strict-aliasing -fPIC conftest.c  -L. -L/usr/lib64 -L. -Wl,--hash-style=gnu -Wl,-O1 -Wl,--as-needed -fno-lto -fstack-protector -rdynamic -Wl,-export-dynamic -Wl,--no-undefined     -lruby25  -lpthread -lgmp -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 */

"x86_64-pc-linux-gnu-gcc -o conftest -I/usr/include/ruby-2.5.0/x86_64-linux -I/usr/include/ruby-2.5.0/ruby/backward -I/usr/include/ruby-2.5.0 -I.    -march=native -O2 -pipe -fno-lto -fno-strict-aliasing -fPIC conftest.c  -L. -L/usr/lib64 -L. -Wl,--hash-style=gnu -Wl,-O1 -Wl,--as-needed -fno-lto -fstack-protector -rdynamic -Wl,-export-dynamic -Wl,--no-undefined     -lruby25 -L/usr/lib64/../lib64 -lffi -lpthread -lgmp -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 libffi
=> "-I/usr/lib64/libffi-3.2.1/include\n"
| pkg-config --cflags-only-other libffi
=> "\n"
| pkg-config --libs-only-l libffi
=> "-lffi\n"
package configuration for libffi
cflags: 
ldflags: -L/usr/lib64/../lib64
libs: -lffi