"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-2.4.0/x86_64-linux -I/usr/include/ruby-2.4.0/ruby/backward -I/usr/include/ruby-2.4.0 -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     -lruby24  -lpthread -lgmp -ldl -lcrypt -lm   -lc"
In file included from /usr/include/ruby-2.4.0/ruby.h:33,
                 from conftest.c:1:
/usr/include/ruby-2.4.0/ruby/ruby.h:631:1: warning: ‘const’ attribute on function returning ‘void’ [-Wattributes]
 CONSTFUNC(void rb_secure_update(VALUE));
 ^~~~~~~~~
In file included from /usr/include/ruby-2.4.0/ruby/ruby.h:2005,
                 from /usr/include/ruby-2.4.0/ruby.h:33,
                 from conftest.c:1:
/usr/include/ruby-2.4.0/ruby/intern.h:257:1: warning: ‘const’ attribute on function returning ‘void’ [-Wattributes]
 CONSTFUNC(void rb_error_untrusted(VALUE));
 ^~~~~~~~~
/usr/include/ruby-2.4.0/ruby/intern.h:259:1: warning: ‘const’ attribute on function returning ‘void’ [-Wattributes]
 CONSTFUNC(void rb_check_trusted(VALUE));
 ^~~~~~~~~
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-2.4.0/x86_64-linux -I/usr/include/ruby-2.4.0/ruby/backward -I/usr/include/ruby-2.4.0 -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     -lruby24 -L/usr/lib64/../lib64 -lffi -lpthread -lgmp -ldl -lcrypt -lm   -lc"
In file included from /usr/include/ruby-2.4.0/ruby.h:33,
                 from conftest.c:1:
/usr/include/ruby-2.4.0/ruby/ruby.h:631:1: warning: ‘const’ attribute on function returning ‘void’ [-Wattributes]
 CONSTFUNC(void rb_secure_update(VALUE));
 ^~~~~~~~~
In file included from /usr/include/ruby-2.4.0/ruby/ruby.h:2005,
                 from /usr/include/ruby-2.4.0/ruby.h:33,
                 from conftest.c:1:
/usr/include/ruby-2.4.0/ruby/intern.h:257:1: warning: ‘const’ attribute on function returning ‘void’ [-Wattributes]
 CONSTFUNC(void rb_error_untrusted(VALUE));
 ^~~~~~~~~
/usr/include/ruby-2.4.0/ruby/intern.h:259:1: warning: ‘const’ attribute on function returning ‘void’ [-Wattributes]
 CONSTFUNC(void rb_check_trusted(VALUE));
 ^~~~~~~~~
checked program was:
/* begin */
1: #include "ruby.h"
2: 
3: int main(int argc, char **argv)
4: {
5:   return 0;
6: }
/* end */

package configuration for libffi is not found