For protocol buffer files, when there were gpb options to alter the
module name with prefix or suffix, recompilation was not properly
detected. This is now fixed. (Issue #384).
Use the rebar_base_compiler's ability to specify both source and
target file names, to be able to also support prefixes.
This also introduces a call to gpb_compile:format_error, so the xref
recipe needs to be updated to ignore it, to avoid false errors.
Make it possible for plug in alternative protocol buffer compilers.
The compilers are picked up based on if they export all of the
functions key/0, proto_compile/3, proto_clean/3 and proto_info/2.
The set of compiler modules to choose from, is fetched from the rebar
application environment, from the app_dir modules.
A new config option, {proto_compiler,Compiler}, specifies which of
the available protocol buffer compilers to use. The 'protobuffs'
compiler is now one such compiler (the only one), and it is also the
default, for backwards compatibility.
