Update APR macro usage to new name-protected names.

Replace REENTRANCY_FLAGS with direct use of THREAD_CPPFLAGS.
This is a temporary band-aid, since the way we currently set
the standard make vars (CFLAGS, CPPFLAGS, LDFLAGS, etc.) is
bogus and frequently redundant [to be fixed later].


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@88245 13f79535-47bb-0310-9956-ffa450edef68