android: Don't emit useless info into generated makefiles.

To make the generated makefiles smaller and slightly easier to debug,
omit information that is useless:

* Don't emit LDFLAGS for static libraries as the linker is not invoked.

* For static libraries, only include system libraries in the lists of
  libraries to link against. The system libraries are required in case
  they provide implicit include paths, but there's no need to include
  the large list of gyp-generated targets.

* Don't emit empty lists of libraries to link against.

* Don't bother setting LOCAL_MODULE_TAGS to optional as that's the
  default value.

Also, stop attempting to remove standard system include paths; the list
is out of date, never matches anything in the Chromium build, and it
doesn't really matter if a standard path is duplicated anyway.

BUG=
R=primiano@chromium.org

Review URL: https://codereview.chromium.org/565883002

git-svn-id: http://gyp.googlecode.com/svn/trunk@1976 78cadc50-ecff-11dd-a971-7dbc132099af
android: Add a way to override build system variables.

Add a key 'aosp_build_settings' which a target can use to set Android
build system variables in the generated makefile, for edge cases not
directly supported by gyp. aosp_build_settings is a dictionary which
maps variable names to values (values may be integers, strings or lists;
all values will be quoted appropriately for make and lists will be
converted to whitespace-separated form).

BUG=
R=primiano@chromium.org, thakis@chromium.org

Review URL: https://codereview.chromium.org/565743004

git-svn-id: http://gyp.googlecode.com/svn/trunk@1977 78cadc50-ecff-11dd-a971-7dbc132099af