makefile with easier customisation

This makefile allows changing the compiler to g++ or requesting debug
binaries without editing the makefile.

The makefile sets default values for CXX and ACDEBUG, then reads values
from the file Makefile_local (if it exists). A commandline parameter
overrules both.

Makefile_local is not part of the repository, so it can be changed without
git caring (or nagging) about it. It is listed in .gitignore.

To switch to g++, create a file "Makefile_local" with the content
"CXX=g++".

To compile debug binaries, type "make clean install ACDEBUG=yes".

When g++ is used, "fast-math" is now enabled.