Third-party configuration scripts may produce undesirable additions to CFLAGS/CPPFLAGS

Third-party configuration scripts may produce undesirable additions to CFLAGS/CPPFLAGS
and LIBS/LDFALGS. In particular, they often don't check where the appropriate libraries
were installed and pass -I/usr/include and -L/usr/lib as part of CFLAGS/LDFLAGS.

While the latter isn't dangerous, passing system include directory through -I lead
to change of its status in CPP from system to user-defined in many cases.

This patch cleans up CFLAGS/CPPFLAGS from errorenous -I/usr/include and LIBS/LDFLAGS
from -L/usr/lib. This is done as two m4 macros which are called before AC_OUTPUT.
(This used to be commit 643172ac1a9b53823d704690eb03120074bbef78)