DynaLoader: Introduce d_libname_unique

Android's linker has some unusual behavior, in that it only uses
the basename of a library in its cache.  That means that, as far as
dlopen() is concerned, the libraries for Hash::Util and List::Util,
both of which are called Util.so, are the same.

This commit teaches DynaLoader about d_libname_unique.  When
defined, it signals DynaLoader define a mod2fname sub that renames
the .so files to something "unique" -- so for example,
Hash/Util/Util.so becomes Hash/Util/PL_Hash__Util.so.
Configure: Introduce d_libname_unique
Teach ExtUtils::CBuilder to handle mod2fname properly