* lib/autoconf/headers.m4 (AC_HEADER_STDC): Don't use AC_TRY_CPP.

* lib/autoconf/headers.m4 (AC_HEADER_STDC): Don't use AC_TRY_CPP.
More `check config.log' messages.
* lib/autoconf/general.m4 (AC_MSG_FAILURE): New.
* doc/autoconf.texi (Printing Messages): Document it.
* lib/autoconf/types.m4 (AC_CHECK_SIZEOF): Use it when
appropriate.
* lib/autoconf/lang.m4 (_AC_COMPILER_EXEEXT_DEFAULT)
(_AC_COMPILER_EXEEXT_WORKS, _AC_COMPILER_EXEEXT_O)
(_AC_COMPILER_OBJEXT): Likewise.
* lib/autoconf/general.m4 (AC_RUN_IFELSE): Likewise.
* lib/autoconf/fortran.m4 (_AC_LANG_PROGRAM_C_F77_HOOKS):
Likewise.
* lib/autoconf/c.m4 (AC_PROG_CPP, AC_PROG_CC, AC_PROG_CXXCPP):
Likewise.
Deprecate macros with unsual interface.
* lib/autoconf/general.m4 (AC_TRY_CPP, AC_TRY_LINK)
(AC_TRY_COMPILE, AC_TRY_RUN): AU_DEFUN'ed.
Document the new ones, and proper style.
* doc/autoconf.texi (Generating Sources): New.
Document AC_LANG_CONFTEST, AC_LANG_SOURCE, AC_LANG_PROGRAM.
(Examining Declarations): Rename as...
(Running the Preprocessor): this.
Document AC_PREPROC_IFELSE.
(Examining Syntax): Rename as...
(Running the Compiler): this.
(AC_FOO_IFELSE vs AC_TRY_FOO): New section.
(Obsolete Macros): Move the definition of AC_TRY_CPP and
AC_TRY_COMPILE here.