Remove support for Interix and SunOS from the Makefile.

- SunOS was replaced with Solaris in 1992(!).
- We have a native Windows port so Interix is not needed.
Tidy up CXXFLAGS and LDFLAGS.

- Add -Iinclude at declaration instead of doing it later on.
- Add -Wextra -Wfatal-errors and -Wno-unused-parameter.
- Enable -Wall and -Wshadow on ICC.
- Remove -DPIC (useless).
- Remove empty declaration of LDFLAGS.
- Remove unneeded CXXFLAGS on Darwin.
Remove 'make squeakyclean'.

This was a non-standard alias for make distclean.
Add IFNEQ Makefile template macro.

- Use template macro for ICC check.
- Don't link against pthread on Darwin.
Use CXX instead of CC.

CXX is the correct variable to use as CC is the C compiler.
Rename CXXFLAGS to CORECXXFLAGS to respect users enviroment.
Merge pull request #577 from SaberUK/master+makefile-cleanup

Tidy up the Makefile template.