Makefile: Cleanup compiler and linker flags

[mniestroj]

Cleanup compiler and linker flags, so the library can be easily
cross-compiled by overwriting CPPFLAGS, CXXFLAGS and LDFLAGS:

• Rename CPP into CXX, as we use this command for compiling, not
  preprocessing.
• Remove separate LD usage, use CXX instead. This is to still launch
  compiler command when user overrides CXX and LD commands.
• Define MY_CXXFLAGS and MY_LDFLAGS, which should always take place and
  not get overridden by user.
• Define AR command, which can be overidden by user.