Android backend: support multiple configurations.

Support building multiple configurations in the Android backend (by
setting GYP_CONFIGURATION when invoking make). Generate both sets of
compile/link flags into the makefile under different names and use the
right one at make time (the same way the make backend works).

Also, simplify cflags processing slightly: the android backend doesn't
differentiate between cflags and cflags_c so just combine them together
at the start of flags processing instead of waiting until make time.

BUG=gyp:349
R=benm@chromium.org, sbc@chromium.org

Review URL: https://codereview.chromium.org/17645002

git-svn-id: http://gyp.googlecode.com/svn/trunk@1654 78cadc50-ecff-11dd-a971-7dbc132099af