Correct SRCLIST variable name.
Preserve CFLAGS & CXXFLAGS for the configure user

The Automake manual makes it clear that certain user variables such as
CC, CFLAGS, CXXFLAGS, CPPFLAGS, and so on are to be preserved for the
user running configure.  This patch cleans up such assignments and
assures that PTHREAD_CFLAGS and so forth are applied to those targets
that require it.