Don't run clean target in parallel as running 'make -j8 clean check' would...

Don't run clean target in parallel as running 'make -j8 clean check' would have some interesting results if check was ran in parallel with clean

Makefile.am

@@ -88,3 +88,4 @@ code-coverage-clean:
 endif
 
 .PHONY: check-code-coverage code-coverage-initial code-coverage-capture code-coverage-html code-coverage-summary code-coverage-clean singlehtml install-singlehtml
+.NOTPARALLEL: clean