This uses the same pattern that is already used to discriminate env.
vars for host and build (i.e., the new option specific for build
platform has -for-build suffix).

It is implemented for Executable, Library and StaticLibrary and works
for C, C++, Objective C and Objective C++ sources.

Please, do not misuse! Better fix your code!

Also rename private subroutines to start with an underscore.

I'm a cruel person, i.e. this targets the TODO of
dab11f7f in
bridges/source/cpp_uno/gcc3_linux_arm/cpp2uno.cx

...and make underlying JavaClassSet implicitly depend on OOO_JUNI_JAR
...and remove unused gb_JunitTest_set_classpath