Adapt sconsx to new compilers in conda

[pradal]

New compilers are distributed with conda (see compiler tools).

When a recipe depends on a compiler, a set of environment variables are set:

• CXX, CXXFLAGS
• CC, CCFLAGS
• GFORTRAN, F95, FC, FFLAGS
• AR, ...

All these flags are ignored by sconsx.
In this issue, we propose to extend the compiler tool in sconsx to configure compiler environment with these values.

[pradal]

This is required to pick the right compiler with its options.
However, while the mapping is done for CMake, we have to find the correct combination for SCons