abstract_compiler: check the presence of pkgconfig libraries in the Makefile

Signed-off-by: Jean Privat <jean@pryen.org>
ffi/pkgconfig: use the new pkgconfig infrastructure in the Makefile

Signed-off-by: Jean Privat <jean@pryen.org>
c_src: update the Maklefile to protect the pkg-config commands

Signed-off-by: Jean Privat <jean@pryen.org>