Remove _glib include path from gi module target
tests: Run PyFlakes and PEP8 only on SUBDIRS

Move PyFlakes and PEP8 checks into check-local of the root Makefile.am. Only
run these tools on selective subdirs of top_srcdir. This fixes a problem
where vpath build directories within the srcdir would contain links to the
source files, causing these tools to run multiple times slowing down the
check process.