scons: Add an analyze option.

For Clang static code analyzer, the scan-build script will produce more
comprehensive output.  Nevertheless you can invoke it as

  CC=clang CXX=clang++ scons analyze=1

For MSVC this is the best way to use its static code analysis.  Simply
invoke as

  scons analyze=1

Reviewed-by: Brian Paul <brianp@vmware.com>