fix: trivial deadlock when hitting memory limit.
fix: make cscope

Pkg-config drawback is that it does not allow to distinguish between
CFLAGS (like -pthread) and CPPFLAGS (like -I/usr/local/include).
We want CPPFLAGS for cscope but not CFLAGS. This patch attempts to filter
out non CPPFLAGS words from CFLAGS using the few available gnu make
function :-/