Added --compiler option and made --config actually work.

SConscript

@@ -127,13 +127,43 @@ AddOption(
     default = 'config.py'
 )
 
+AddOption(
+    '--compiler',
+    dest = 'compiler',
+    type = 'choice',
+    choices = ('auto', 'gcc', 'clang', 'msvc'),
+    nargs = 1,
+    action = 'store',
+    default = 'auto'
+)
+
 build_type      = GetOption('build_type')
 unity_mode      = GetOption('unity_mode')
 variant         = GetOption('variant')
 enable_asan     = GetOption('enable_asan')
 config_file     = GetOption('config_file')
+compiler        = GetOption('compiler')
 
-env = Environment(tools = ['default', 'compilation_db', 'unity_build'])
+default_CC = {
+    'gcc': 'gcc',
+    'clang': 'clang',
+    'msvc': 'cl.exe'
+}.get(compiler, None)
+default_CXX = {
+    'gcc': 'g++',
+    'clang': 'clang++',
+    'msvc': 'cl.exe'
+}.get(compiler, None)
+
+vars = Variables(config_file)
+vars.Add('CC', 'The C Compiler', default_CC)
+vars.Add('CXX', 'The C++ Compiler', default_CXX)
+vars.Add('LINK', 'The Linker')
+vars.Add('CCFLAGS', 'C/C++ Compiler Flags')
+vars.Add('LINKFLAGS', 'Linker Flags')
+vars.Add('PYTHON', 'Python Executable', 'python')
+
+env = Environment(tools = ['default', 'compilation_db', 'unity_build'], variables = vars)
 env['RECIPES_FOLDERS'] = [Dir('recipes')]
 env['SYSTEM_CACHE_DIR'] = os.path.join(_find_system_cache_dir(), 'spp_cache')
 env['CLONE_DIR'] = os.path.join(env['SYSTEM_CACHE_DIR'], 'cloned')