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>

Makefile.am

@@ -162,9 +162,9 @@ uninstall-local:
 CLEANFILES= $(GENERATED) $(QEMU_GENERATED) $(LXC_GENERATED) $(GENERATE).stamp \
 	*.pyc
 
-else
+else ! WITH_PYTHON
 all:
-endif
+endif ! WITH_PYTHON
 
 dummy: