meson: fix cross-compilation issues with gperf header file preprocessing

Pass c_args to the compiler when preprocessing the gperf header file,
they might contain important bits without which compilation/preprocessing
might fail (e.g. with clang on Android). cc.cmd_array() does not include
the c_args and we can't easily look them up from the meson.build file, so
we have to retrieve from the introspection info.

This is basically the Meson equivalent to commit 57103773.

src/cutout.py

@@ -1,5 +1,6 @@
 import argparse
 import subprocess
+import json
 import os
 import re
 
@@ -7,12 +8,23 @@ if __name__== '__main__':
     parser = argparse.ArgumentParser()
     parser.add_argument('input')
     parser.add_argument('output')
+    parser.add_argument('buildroot')
 
     args = parser.parse_known_args()
-    print (args[0].output)
+
+    # c_args might contain things that are essential for crosscompilation, but
+    # are not included in cc.cmd_array(), so we have to look them up ourselves
+    host_cargs = []
+    buildroot = args[0].buildroot
+    with open(os.path.join(buildroot, 'meson-info', 'intro-buildoptions.json')) as json_file:
+        bopts = json.load(json_file)
+        for opt in bopts:
+            if opt['name'] == 'c_args' and opt['section'] == 'compiler' and opt['machine'] == 'host':
+                host_cargs = opt['value']
+                break
 
     cpp = args[1]
-    ret = subprocess.run(cpp + [args[0].input], stdout=subprocess.PIPE, check=True)
+    ret = subprocess.run(cpp + host_cargs + [args[0].input], stdout=subprocess.PIPE, check=True)
 
     stdout = ret.stdout.decode('utf8')
 

src/meson.build

@@ -44,10 +44,13 @@ endif
 
 cpp += ['-I', join_paths(meson.current_source_dir(), '..')]
 
+# Can use meson.project_build_dir() once we require Meson 0.56
+project_build_dir = meson.current_build_dir() / '..'
+
 fcobjshash_gperf = custom_target('fcobjshash.gperf',
   input: 'fcobjshash.gperf.h',
   output: 'fcobjshash.gperf',
-  command: [python3, files('cutout.py')[0], '@INPUT@', '@OUTPUT@', cpp],
+  command: [python3, files('cutout.py')[0], '@INPUT@', '@OUTPUT@', project_build_dir, cpp],
   build_by_default: true,
 )