syntax-check: Add all target

The makefile is an implementation detail, so point users towards
the proper way of running syntax-check if they happen to call it
directly.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>

build-aux/syntax-check.mk

@@ -20,6 +20,11 @@
 # <http://www.gnu.org/licenses/>.
 
 
+all:
+	@echo "Do not call this file directly, use 'meson test' instead" >&2; \
+	exit 1
+
+
 ## ----- ##
 ## Rules ##
 ## ----- ##