autoconf: port 'restrict' to GCC 4.2.1

* lib/autoconf/c.m4 (AC_C_RESTRICT): Detect GCC bug 14050.
Problem reported by Marco Munari for OpenBSD 5.5.
autoconf: fix typo in previous change

* lib/autoconf/c.m4 (AC_C_RESTRICT): Fix typo in previous change.
autoconf: clarify "checking" message for GNU-compatible compiliers

Problem reported by Bastien Chevreux in:
http://lists.gnu.org/archive/html/autoconf/2014-09/msg00022.html
and idea for fix by Eric Blake in:
http://lists.gnu.org/archive/html/autoconf/2014-09/msg00025.html
* doc/autoconf.texi (Running the Preprocessor)
(Present But Cannot Be Compiled):
Adjust examples to match current behavior.
* lib/autoconf/lang.m4 (_AC_LANG_COMPILER_GNU): Say
"checking whether the compiler supports GNU C", not
"checking whether we are using the GNU C compiler".
doc: distinguish GCC from GNU C, etc.

* doc/autoconf.texi (C Compiler, Objective C Compiler)
(Objective C++ Compiler, Fortran Compiler): Be more careful about
distinguishing GNU C from GCC, and similarly for other languages.
Problem reported by Marko Lindqvist in:
http://lists.gnu.org/archive/html/autoconf/2014-09/msg00027.html