autoconf: warn if AC_*_IFELSE lacks complete program

autoconf: warn if AC_*_IFELSE lacks complete program

* lib/autoconf/lang.m4 (AC_LANG_DEFINES_PROVIDED): New macro.
(AC_LANG_SOURCE): Call it.
(AC_LANG_CONFTEST): Add warning if new macro is not called.
* lib/autoconf/c.m4 (_AC_LANG_OPENMP): Add missing AC_LANG_SOURCE.
* lib/autoconf/fortran.m4 (AC_FC_FREEFORM, AC_FC_FIXEDFORM)
(AC_FC_LINE_LENGTH, __AC_FC_NAME_MANGLING): Intentionally bypass
AC_LANG_SOURCE.
* lib/autoconf/programs.m4 (_AC_PROG_LEX_YYTEXT_DECL): Likewise.
* tests/compile.at (AC_COMPILE_IFELSE): New test.
* doc/autoconf.texi (Generating Sources) <AC_LANG_CONFTEST>:
Document new warning.
<AC_LANG_DEFINES_PROVIDED>: Document new macro.
<AC_LANG_SOURCE>: Document use of new macro.
* NEWS: Document the improvement.
Suggested by Bruno Haible.

Signed-off-by: Eric Blake <eblake@redhat.com>