+ Use the uname convention from Git's makefile to detect the OS.

+ Use the standard OBJS variable so object file names are listed only once.

+ Make sure clean removes via OBJS.

+ Removed -mimpure-text flag as it is Solaris-only
  (see http://gcc.gnu.org/onlinedocs/gcc/Option-Summary.html)
  Older versions of GCC were more lenient of it's presence.
  Breaks under GCC 4.7.0 or later.

+ Make sure a jnilib is generated only on Darwin and so's are generated only
  on Linux.

Signed-off-by: Michael Pellon <michael@p3ll0n.net>

- Additional uname macros for determining underlying arch.

- New GCPROF_LIB macro that is generated as a .so or .jnilib depending
  on the underlying platform. Architecture and platform is also encoded
  in the filename.

- Comments added to improve readability.