msetup: use more consistent exceptions on exit

- MesonException for errors is clearer than SystemExit('error message')
  and provides meson-formatted "ERROR: ..."

- `raise SystemExit` with no parameter isn't obvious that it intends to
  exit successfully

While clarifying the latter, it was observed to cause
test_preprocessor_checks_CPPFLAGS() failure to be ignored. That test
checks get_define() on both c and cpp compilers, which means we need to
define either CPPFLAGS or both CFLAGS+CXXFLAGS.

mesonbuild/msetup.py

@@ -165,16 +165,21 @@ class MesonApp:
         has_partial_build = os.path.isdir(priv_dir)
         if has_valid_build:
             if not reconfigure and not wipe:
-                print('Directory already configured.\n'
-                      '\nJust run your build command (e.g. ninja) and Meson will regenerate as necessary.\n'
+                print('Directory already configured.\n\n'
+                      'Just run your build command (e.g. ninja) and Meson will regenerate as necessary.\n'
                       'If ninja fails, run "ninja reconfigure" or "meson setup --reconfigure"\n'
-                      'to force Meson to regenerate.\n'
-                      '\nIf build failures persist, run "meson setup --wipe" to rebuild from scratch\n'
-                      'using the same options as passed when configuring the build.'
-                      '\nTo change option values, run "meson configure" instead.')
-                raise SystemExit
+                      'to force Meson to regenerate.\n\n'
+                      'If build failures persist, run "meson setup --wipe" to rebuild from scratch\n'
+                      'using the same options as passed when configuring the build.\n'
+                      'To change option values, run "meson configure" instead.')
+                # FIXME: This returns success and ignores new option values from CLI.
+                # We should either make this a hard error, or update options and
+                # return success.
+                # Note that making this an error would not be backward compatible (and also isn't
+                # universally agreed on): https://github.com/mesonbuild/meson/pull/4249.
+                raise SystemExit(0)
         elif not has_partial_build and 'MESON_RUNNING_IN_PROJECT_TESTS' not in os.environ:
-            raise SystemExit(f'Directory is not empty and does not contain a previous build:\n{build_dir}')
+            raise MesonException(f'Directory is not empty and does not contain a previous build:\n{build_dir}')
         return src_dir, build_dir
 
     def generate(self) -> None:

unittests/allplatformstests.py

@@ -1139,7 +1139,7 @@ class AllPlatformTests(BasePlatformTests):
 
     def test_preprocessor_checks_CPPFLAGS(self):
         '''
-        Test that preprocessor compiler checks read CPPFLAGS and also CFLAGS but
+        Test that preprocessor compiler checks read CPPFLAGS and also CFLAGS/CXXFLAGS but
         not LDFLAGS.
         '''
         testdir = os.path.join(self.common_test_dir, '132 get define')
@@ -1150,11 +1150,13 @@ class AllPlatformTests(BasePlatformTests):
         # % and # confuse the MSVC preprocessor
         # !, ^, *, and < confuse lcc preprocessor
         value = 'spaces and fun@$&()-=_+{}[]:;>?,./~`'
-        for env_var in ['CPPFLAGS', 'CFLAGS']:
+        for env_var in [{'CPPFLAGS'}, {'CFLAGS', 'CXXFLAGS'}]:
             env = {}
-            env[env_var] = f'-D{define}="{value}"'
+            for i in env_var:
+                env[i] = f'-D{define}="{value}"'
             env['LDFLAGS'] = '-DMESON_FAIL_VALUE=cflags-read'
             self.init(testdir, extra_args=[f'-D{define}={value}'], override_envvars=env)
+            self.new_builddir()
 
     def test_custom_target_exe_data_deterministic(self):
         testdir = os.path.join(self.common_test_dir, '109 custom target capture')