Simplify the interface: users shouldn't need to explicitly check

Simplify the interface: users shouldn't need to explicitly check
for special environments.

* acspecific.m4 (AC_EXEEXT, AC_OBJEXT): AU defined to
nothing.  Replace them by...
(_AC_EXEEXT, _AC_OBJEXT): this.
* aclang.m4 (AC_PROG_CC, AC_PROG_CXX, AC_PROG_F77): Call them.
(AC_PROG_CC_G, AC_PROG_CXX_G, AC_PROG_F77_G): Rename as...
(_AC_PROG_CC_G, _AC_PROG_CXX_G, _AC_PROG_F77_G): this.
Adjust dependencies.
* tests/Makefile.am (FILTER_MACROS): Adjust.
* doc/autoconf.texi (Compilers and Preprocessors): New section,
move the documentation of AC_PROG_CC, AC_PROG_CC_C_O,
AC_PROG_CC_STDC, AC_PROG_CPP, AC_PROG_CXX, AC_PROG_CXXCPP,
AC_PROG_F77, AC_PROG_F77_C_O, AC_PROG_GCC_TRADITIONAL here.
Factor the comment documentation of AC_PROG_CC, AC_PROG_CXX,
AC_PROG_F77.