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.
Remove unneeded CXXFLAGS on Darwin.
Remove 'make squeakyclean'.

This was a non-standard alias for make distclean.
Tidy up CXXFLAGS.

- Add -Wextra -Wfatal-errors and -Wno-unused-parameter.
- Enable -Wall and -Wshadow on ICC.
- Remove -DPIC (useless).
Add IFNEQ Makefile template macro.

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

CXX is the correct variable to use as CC is the C compiler.
Use the correct GNU C++ compiler binary name.