Use compiler detector in detect_system_compiler()

Use the compiler detector in detect_system_compiler(), rather than trying to
guess based on what it is the PATH (which could utterly fail e.g when CC env
var is set)

Note that this detection is only used by platform_fix_name() to interpret
installed_files.txt

run_project_tests.py

@@ -132,17 +132,17 @@ def platform_fix_name(fname, compiler, env):
 
     if fname.startswith('?msvc:'):
         fname = fname[6:]
-        if compiler != 'cl':
+        if compiler != 'msvc':
             return None
 
     if fname.startswith('?gcc:'):
         fname = fname[5:]
-        if compiler == 'cl':
+        if compiler == 'msvc':
             return None
 
     if fname.startswith('?cygwin:'):
         fname = fname[8:]
-        if compiler == 'cl' or not mesonlib.for_cygwin(env.is_cross_build(), env):
+        if compiler == 'msvc' or not mesonlib.for_cygwin(env.is_cross_build(), env):
             return None
 
     return fname
@@ -687,14 +687,18 @@ def check_meson_commands_work():
 
 def detect_system_compiler():
     global system_compiler
-    if shutil.which('cl'):
-        system_compiler = 'cl'
-    elif shutil.which('cc'):
-        system_compiler = 'cc'
-    elif shutil.which('gcc'):
-        system_compiler = 'gcc'
-    else:
-        raise RuntimeError("Could not find C compiler.")
+
+    with AutoDeletedDir(tempfile.mkdtemp(prefix='b ', dir='.')) as build_dir:
+        env = environment.Environment(None, build_dir, get_fake_options('/'))
+        try:
+            comp = env.detect_c_compiler(env.is_cross_build())
+        except:
+            raise RuntimeError("Could not find C compiler.")
+        system_compiler = comp.get_id()
+
+    # canonicalize for platform_fix_name()
+    if system_compiler == 'clang-cl':
+        system_compiler = 'msvc'
 
 if __name__ == '__main__':
     parser = argparse.ArgumentParser(description="Run the test suite of Meson.")