Limit stderr logging for C compiler version.

Limit stderr logging for C compiler version.

* lib/autoconf/general.m4 (_AC_RUN_LOG_LIMIT, _AC_DO_LIMIT): New
internal macros, equivalent to _AC_RUN_LOG and _AC_DO, but with
an optional additional argument to limit the number of lines of
stderr output logged, defaulting to 10.
* lib/autoconf/c.m4 (AC_PROG_CC, AC_PROG_CXX, AC_PROG_OBJC): Use
_AC_DO_LIMIT for capturing compiler version output.  Also test
-qversion, for the IBM xlc compiler.
* lib/autoconf/fortran.m4 (_AC_PROG_FC): Likewise.
* THANKS: Update.
Report by Christian Rössel and John R. Cary against Libtool.

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