Revert "Revert "compilerplugins: Build them in parallel.""

This reverts commit cd2725de.

Norbert's list of CI performance issues includes
"2/ clang plugin build should not be serialized (forced -j1)"

Change-Id: Ib77f951a31adc20f6a9f88e8b51632bd81273327
Reviewed-on: https://gerrit.libreoffice.org/42595Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Michael Stahl <mstahl@redhat.com>

compilerplugins/Makefile.mk

@@ -21,7 +21,11 @@ else
 
 ifeq ($(COM_IS_CLANG),TRUE)
 
-include $(SRCDIR)/compilerplugins/Makefile-clang.mk
+compilerplugins:
+	$(MAKE) $(PARALLELISM_OPTION) -f $(SRCDIR)/compilerplugins/Makefile-clang.mk compilerplugins
+
+compilerplugins-clean:
+	$(MAKE) -f $(SRCDIR)/compilerplugins/Makefile-clang.mk compilerplugins-clean
 
 compilerplugins.clean: compilerplugins-clean