Add a way to declare explicitly the path to a C compiler, and update haveacompiler()
Reported by Samuel GOUGEON (@sgougeon)
Bug description:
The detection or declaration of a C compiler is needed in many parts of Scilab,
but is plateform-dependent, not always reliable, or sometimes quite tricky.
It would be simpler as well as more reliable to be able to declare the
path to a C compiler explicitly in the Preferences, adapt haveacompiler(),
and simplify all the stuff such as findmsvccompiler() (whose maintenance is continuous at each new release of the targetted specific class of compilers),
chooselcccompiler() (already moved to ATOMS), configure_msvc() that is also very specific, etc.
A tab or a frame in the Preferences should be dedicated to compilation.
Later, the same could be implemented for a Fortran compiler.