Environment: Fix passing envrionment variables CPPFLAGS and CFLAGS

Or other language flags that use CPPFLAGS (like CXXFLAGS). The problem
here is actually rather simple, `dict.setdefault()` doesn't work like I
thought it did, I thought it created a weak entry, but it actually is
equivalent to:
```python
if k not in dict:
    dict[k] = v
```
Instead we'll use an intermediate dictionary (a default dictionary
actually, since that makes things a little cleaner) and then add the
keys from that dict to self.options as applicable.

Test case written by Jussi, Fix by Dylan

Co-authored-by: Jussi Pakkanen
Fixes: #8361
Fixes: #8345

mesonbuild/environment.py

@@ -788,6 +788,8 @@ class Environment:
             ]
         )
 
+        env_opts: T.DefaultDict[OptionKey, T.List[str]] = collections.defaultdict(list)
+
         for (evar, keyname), for_machine in itertools.product(opts, MachineChoice):
             p_env = _get_env_var(for_machine, self.is_cross_build(), evar)
             if p_env is not None:
@@ -805,7 +807,7 @@ class Environment:
                     p_list = list(mesonlib.OrderedSet(p_env.split(':')))
                 else:
                     p_list = split_args(p_env)
-                p_list = [e for e in p_list if e]  # filter out any empty eelemnts
+                p_list = [e for e in p_list if e]  # filter out any empty elements
 
                 # Take env vars only on first invocation, if the env changes when
                 # reconfiguring it gets ignored.
@@ -816,18 +818,21 @@ class Environment:
                         key = OptionKey('link_args', machine=for_machine, lang='c')  # needs a language to initialize properly
                         for lang in compilers.compilers.LANGUAGES_USING_LDFLAGS:
                             key = key.evolve(lang=lang)
-                            v = mesonlib.listify(self.options.get(key, []))
-                            self.options.setdefault(key, v + p_list)
+                            env_opts[key].extend(p_list)
                     elif keyname == 'cppflags':
                         key = OptionKey('args', machine=for_machine, lang='c')
                         for lang in compilers.compilers.LANGUAGES_USING_CPPFLAGS:
                             key = key.evolve(lang=lang)
-                            v = mesonlib.listify(self.options.get(key, []))
-                            self.options.setdefault(key, v + p_list)
+                            env_opts[key].extend(p_list)
                     else:
                         key = OptionKey.from_string(keyname).evolve(machine=for_machine)
-                        v = mesonlib.listify(self.options.get(key, []))
-                        self.options.setdefault(key, v + p_list)
+                        env_opts[key].extend(p_list)
+
+        # Only store options that are not already in self.options,
+        # otherwise we'd override the machine files
+        for k, v in env_opts.items():
+            if k not in self.options:
+                self.options[k] = v
 
     def _set_default_binaries_from_env(self) -> None:
         """Set default binaries from the environment.

mesonbuild/mesonlib/universal.py

@@ -2023,7 +2023,6 @@ class OptionKey:
         This takes strings like `mysubproject:build.myoption` and Creates an
         OptionKey out of them.
         """
-
         try:
             subproject, raw2 = raw.split(':')
         except ValueError:

run_unittests.py

@@ -5476,6 +5476,14 @@ class AllPlatformTests(BasePlatformTests):
         projinfo = self.introspect('--projectinfo')
         self.assertEqual(projinfo['version'], '1.0.0')
 
+    def test_cflags_cppflags(self):
+        envs = {'CPPFLAGS': '-DCPPFLAG',
+                'CFLAGS': '-DCFLAG',
+                'CXXFLAGS': '-DCXXFLAG'}
+        srcdir = os.path.join(self.unit_test_dir, '90 multiple envvars')
+        self.init(srcdir, override_envvars=envs)
+        self.build()
+
 
 class FailureTests(BasePlatformTests):
     '''

test cases/unit/90 multiple envvars/meson.build

@@ -0,0 +1,4 @@
+project('multienv', 'c', 'cpp')
+
+executable('cexe', 'prog.c')
+executable('cppexe', 'prog.cpp')

test cases/unit/90 multiple envvars/prog.c

@@ -0,0 +1,18 @@
+#include<stdio.h>
+
+#ifndef CPPFLAG
+#error CPPFLAG not set
+#endif
+
+#ifndef CFLAG
+#error CFLAGS not set
+#endif
+
+#ifdef CXXFLAG
+#error CXXFLAG is set
+#endif
+
+int main(int argc, char **argv) {
+    printf("%d %s\n", argc, argv[0]);
+    return 0;
+}

test cases/unit/90 multiple envvars/prog.cpp

@@ -0,0 +1,18 @@
+#include<cstdio>
+
+#ifndef CPPFLAG
+#error CPPFLAG not set
+#endif
+
+#ifdef CFLAG
+#error CFLAG is set
+#endif
+
+#ifndef CXXFLAG
+#error CXXFLAG not set
+#endif
+
+int main(int argc, char **argv) {
+    printf("%d %s\n", argc, argv[0]);
+    return 0;
+}