build: enforce makefile conditional style

Automake has builtin support to prevent botched conditional nesting,
but only if you use:
if FOO
else !FOO
endif !FOO

An example error message when using the wrong name:

daemon/Makefile.am:378: error: else reminder (LIBVIRT_INIT_SCRIPT_SYSTEMD_TRUE) incompatible with current conditional: LIBVIRT_INIT_SCRIPT_SYSTEMD_FALSE
daemon/Makefile.am:381: error: endif reminder (LIBVIRT_INIT_SCRIPT_SYSTEMD_TRUE) incompatible with current conditional: LIBVIRT_INIT_SCRIPT_SYSTEMD_FALSE

As our makefiles tend to have quite a bit of nested conditionals,
it's better to take advantage of the benefits of the build system
double-checking that our conditionals are well-nested, but that
requires a syntax check to enforce our usage style.

Alas, unlike C preprocessor and spec files, we can't use indentation
to make it easier to see how deeply nesting goes.

* cfg.mk (sc_makefile_conditionals): New rule.
* daemon/Makefile.am: Enforce the style.
* gnulib/tests/Makefile.am: Likewise.
* python/Makefile.am: Likewise.
* src/Makefile.am: Likewise.
* tests/Makefile.am: Likewise.
* tools/Makefile.am: Likewise.

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

gnulib/tests/Makefile.am

@@ -26,7 +26,7 @@ if WITH_EXPENSIVE_TESTS
 ## Automake requires that at least one conditional call out all tests to
 ## be run, for those tests to be shipped in the tarball
 TESTS = $(GNULIB_TESTS)
-endif
+endif WITH_EXPENSIVE_TESTS
 ## However, we want to change the set of tests based on the make environment,
 ## where the default was set at configure time.  Use GNU make constructs to
 ## hide our actions from Automake, so we don't get it too confused.