Revert "Merge pull request #386 from tomas-abrahamsson/gpb-recompilation-detection"

This reverts commit 81063d33f0939416f9bc0a7fc6a4cc326a0059d2, reversing
changes made to 6584defdda209d43505baee21b0f9f4d7f829573.

This was reverted because it changed the gpb compiler to
not use rebar_base_compiler.  That change was to add support for
target name prefixes, but it is better to add that support to
rebar_base_compiler.
Merge pull request #413 from tomas-abrahamsson/undo-386-merge

Revert "Merge pull request #386 ... gpb-recompilation...detection"