Fix Makefile

ldm fails to build due to Makefile related issue. Here's the error log: https://gist.github.com/asergi/8038209

In this PR:
Use proper operators for variable assignments.
Use parentheses to surround variables instead of curly braces.
Add uninstall target.
General clean and improvements.

For Arch users, the trick (export LDFLAGS="") in the PKGBUILD is no longer needed.