Invoke check_headers with $(SHELL)

Otherwise, it gets executed with /bin/sh which on Solaris does not support the
$(...) construct. This causes 'gmake check' to fail. For example:

../src/check_headers: $(dirname ../src/check_headers): does not exist

Calling check_headers with $(SHELL) explicitly makes use of CONFIG_SHELL.
Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>

lib/Makefile.am

@@ -75,4 +75,4 @@ pkgconfig_DATA = libvlc.pc
 	$(SHELL) ./config.status --file="lib/$@"
 
 check-local:
-	$(top_srcdir)/src/check_headers $(pkginclude_HEADERS)
+	$(SHELL) $(top_srcdir)/src/check_headers $(pkginclude_HEADERS)

src/Makefile.am

@@ -531,7 +531,7 @@ check-local:
 			exit 1; \
 		fi ; \
 	done
-	$(srcdir)/check_headers $(pluginsinclude_HEADERS)
+	$(SHELL) $(srcdir)/check_headers $(pluginsinclude_HEADERS)
 
 FORCE:
 	@echo "Generated source cannot be phony. Go away." >&2