Save/restore AC_LANG setting in AC_PROG_CC_C_O.

Save/restore AC_LANG setting in AC_PROG_CC_C_O.

All of the AC_PROG_{compiler}_C_O macros, *except* AC_PROG_CC_C_O,
wrap their test in an AC_LANG_PUSH/POP pair for the appropriate
language.  https://savannah.gnu.org/support/index.php?110987 reports
that AC_PROG_CC_C_O’s not doing this can cause a configure script to
malfunction.  Specifically, if a script invokes AC_PROG_CC_C_O with
AC_LANG([C++]) in effect, AC_PROG_CC_C_O still runs $CC but it passes
a .cpp source file, possibly along with C-specific options.  Either
of those can cause the test to mis-detect support for -c -o.

Add an AC_LANG_PUSH/POP pair to AC_PROG_CC_C_O, ensuring we are
testing the right compiler and giving it appropriate input.

* lib/autoconf/c.m4 (AC_PROG_CC_C_O): Wrap body of test in
  AC_LANG_PUSH/POP to C.