detect_c_compiler(): support extra commands and arguments in the string

- Hanlde correctly a multi command string in evironment variable, e.g.:
  CC="ccache gcc" meson
- Handle correctly a list for the cross-file option, e.g:
  [binaries]
  c = ['ccache', '/usr/local/bin/mips-linuc-gcc']

This commit fixes mesonbuild#1392.

mesonbuild/environment.py

@@ -349,7 +349,9 @@ def get_gnu_compiler_type(defines):
     def detect_c_compiler(self, want_cross):
         evar = 'CC'
         if self.is_cross_build() and want_cross:
-            compilers = [self.cross_info.config['binaries']['c']]
+            compilers = self.cross_info.config['binaries']['c']
+            if not isinstance(compilers, list):
+                compilers = [compilers]
             ccache = []
             is_cross = True
             if self.cross_info.need_exe_wrapper():
@@ -386,22 +388,22 @@ def detect_c_compiler(self, want_cross):
                     continue
                 gtype = self.get_gnu_compiler_type(defines)
                 version = self.get_gnu_version_from_defines(defines)
-                return GnuCCompiler(ccache + [compiler], version, gtype, is_cross, exe_wrap, defines)
+                return GnuCCompiler(ccache + compilers, version, gtype, is_cross, exe_wrap, defines)
             if 'clang' in out:
                 if 'Apple' in out or for_darwin(want_cross, self):
                     cltype = CLANG_OSX
                 else:
                     cltype = CLANG_STANDARD
-                return ClangCCompiler(ccache + [compiler], version, cltype, is_cross, exe_wrap)
+                return ClangCCompiler(ccache + compilers, version, cltype, is_cross, exe_wrap)
             if 'Microsoft' in out or 'Microsoft' in err:
                 # Visual Studio prints version number to stderr but
                 # everything else to stdout. Why? Lord only knows.
                 version = search_version(err)
-                return VisualStudioCCompiler([compiler], version, is_cross, exe_wrap)
+                return VisualStudioCCompiler(compilers, version, is_cross, exe_wrap)
             if '(ICC)' in out:
                 # TODO: add microsoft add check OSX
                 inteltype = ICC_STANDARD
-                return IntelCCompiler(ccache + [compiler], version, inteltype, is_cross, exe_wrap)
+                return IntelCCompiler(ccache + compilers, version, inteltype, is_cross, exe_wrap)
         errmsg = 'Unknown compiler(s): "' + ', '.join(compilers) + '"'
         if popen_exceptions:
             errmsg += '\nThe follow exceptions were encountered:'