#1 sffview: Support standard Makefile variables

[Olly Betts]

The attached patch adjusts the sffview Makefile to support the standard variable names:

CXX
CPPFLAGS
CXXFLAGS
LDFLAGS

Supporting these standard variables is easier for users as the names they expect to work will now work, and it means distro package tooling will often automatically do the right thing - e.g. Debian's tooling defaults to passing hardening flags in CPPFLAGS, CXXFLAGS and LDFLAGS, and for cross-building sets CC to the C compiler and CXX to the C++ compiler (so using CC for the C++ compiler as the Makefile currently does is unhelpful).

When none of these variables are overridden, the only difference is that $(CXXFLAGS) is now passed when linking, since that's what's expected. The default CXXFLAGS are -g -Os which have no effect when linking, so the behaviour should be unchanged.

[Peter Schaefer-Hutter]

Patch applied. Thanks!