apache2: cleanup buildpaths for target stuffs

Those buildpaths were generated from configure substitutions, they are
required for cross-compiling, but obviously they should be cleaned up
from target stuffs.

Cleanup buildpaths from config_vars.mk and config.nice:

* remove ${STAGING_DIR_HOST} from CC, CFLAGS ...
* set APU_INCLUDEDIR, APU_CONFIG as empty
* remove buildpath from configure line

Signed-off-by: Wenzong Fan <wenzong.fan@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>