Make the compiler used by Makefile.devel customizable. Reported by Mike Fulton <mikefultonpersonal@gmail.com> in <https://lists.gnu.org/archive/html/bug-gnu-libiconv/2023-05/msg00006.html>. * Makefile.devel: Require GNU make. (CC): Use value from the environment, if set and non-empty. (CFLAGS): Use value from the environment, if set. * autogen.sh (GMAKE): New variable. Use it for executing Makefile.devel.