lib: use shorter way to test if variable is set

lib: use shorter way to test if variable is set

Based on an idea by Bernhard Reutner-Fischer.

We frequently used the idiom of 'test "${var+set}" = set' to
test if $var was set to a non-empty string, but this can portably
be trimmed to a more compact 'test ${var+y}' for a smaller
configure file.  Testing that a variable is not set can be done
with '${var+false} :' (although the value of $? is not reliably
1 when the variable is set).

The code for AS_VAR_TEST_SET already used the form '${var+:} false',
but it is slightly longer, and does not guarantee $? of 1.

Tested on coreutils, where the resulting configure file is about
1k smaller.

* doc/autoconf.texi (Shell Substitutions): Prefer shorter sequence
for testing if a variable is set.
(Limitations of Builtins) <test (strings)>: Document it.
* configure.ac: Use it.
* lib/autoconf/c.m4 (_AC_PROG_CC_G, _AC_PROG_CXX_G)
(_AC_PROG_OBJC_G, _AC_PROG_OBJCXX_G): Likewise.
* lib/autoconf/fortran.m4 (_AC_PROG_FC_G): Likewise.
* lib/autoconf/general.m4 (_AC_ENABLE_IF_ACTION, AC_CACHE_SAVE):
Likewise.
* lib/autoconf/lang.m4 (_AC_COMPILER_EXEEXT_DEFAULT): Likewise.
* lib/autoconf/programs.m4 (AC_PROG_INSTALL, AC_PROG_MKDIR_P)
(_AC_PROG_LEX_YYTEXT_DECL): Likewise.
* lib/autoconf/status.m4 (_AC_OUTPUT_MAIN_LOOP): Likewise.
* lib/autotest/general.m4 (AT_INIT): Likewise.
* tests/base.at (AC_CACHE_CHECK): Likewise.
* tests/m4sh.at (LINENO): Likewise.
* lib/m4sugar/m4sh.m4 (_AS_BOURNE_COMPATIBLE)
(_AS_DETECT_BETTER_SHELL, _AS_SHELL_SANITIZE)
(_AS_PATH_SEPARATOR_PREPARE): Likewise.
(AS_VAR_TEST_SET): Use shorter sequence.

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