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.
3 files changed