be more specific when searching for evhtp

Rather than setting blanket CFLAGS and LIBS that add evhtp to every
single compile or link command, add specific evhtp_CFLAGS and evhtp_LIBS
variables and include them only where needed.

This prepares the way for a smooth migration to use pkg-config to find
libevhtp (available in 1.2.11 but gclient currently pulls in 1.2.10).

Author: philandstuff