PATCH: cmake files - Bug correction (for x64 MSVC compilation) & Improvements for MSVC compilation

This patch corrects a major bug for x64 MSVC compilation, when forcing to
compile assembler source with the x86 Macro Assembler (ml.exe) : really check
that the VCINSTALLDIR environment variable is defined (references :
http://www.cmake.org/pipermail/cmake/2011-October/046698.html and
http://www.cmake.org/pipermail/cmake/2011-October/046704.html). This bug arose
at boot sectors compilation.

This patch also introduces some improvements for MSVC compilation :

• Use variables encrypting the used compiler instead of using hardcoded values.
• Same modification for the Message Compiler (this already exists for GCC
  compilation).