Fix AC_OPENMP configure message for non-C compilers.

Fix AC_OPENMP configure message for non-C compilers.

* lib/autoconf/lang.m4 (AC_LANG_DEFINE): Accept as additional
fourth arg the compiler variable name, defined in _AC_CC($1).
(_AC_CC): New language dispatch macro.
* lib/autoconf/erlang.m4 (AC_LANG(Erlang)): Adjust.
* lib/autoconf/fortran.m4 (AC_LANG(Fortran 77), AC_LANG(Fortran)):
Likewise.
* lib/autoconf/c.m4 (AC_LANG(C), AC_LANG(C++))
(AC_LANG(Objective C), AC_LANG(Objective C++)): Likewise.
(AC_OPENMP): Use _AC_CC instead of $CC.

Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>