Inject STL_INCLUDES via MODULE_INCLUDES for the gmp plugin

This avoids having to rely on manual modifications to targets.mk,
which are overwritten if the targets.mk files are regenerated
via the script.
Make sure that libgmpopenh264.so is removed on "make clean"

Nothing used the makefile variable $(PLUGINS), and $(LIBRARIES)
is only used for the clean target.
Use a less generic name for linking to STL when building the plugin for android

This matches the common naming scheme more closely for adding extra
flags to individual parts of the build.

Also deduplicate the static stlport lib name.
Don't build an unused libmodule.a file

This also avoids another manual edit in module/targets.mk. Now
the file is completely autogenerated by the build/mktargets.sh
script, just as all the other targets.mk files.
Merge pull request #1008 from mstorsjo/plugin-build

Clean up minor aspects of the plugin build process