meson: use wrapper program instead of bypassing it

meson persists its own full path (argv[0]) to the build.ninja file, which then
runs it without the PYTHONPATH which the meson wrapper program sets up.

To make this work, we need to make meson write the full path to its *wrapper
program* into build.ninja.

pkgs/meson/build.textproto

@@ -1,6 +1,6 @@
 source: "https://github.com/mesonbuild/meson/releases/download/0.54.1/meson-0.54.1.tar.gz"
 hash: "2f76fb4572762be13ee479292610091b4509af5788bcceb391fe222bcd0296dc"
-version: "0.54.1-5"
+version: "0.54.1-7"
 
 writable_sourcedir: true
 in_tree_build: true
@@ -9,6 +9,8 @@ cherry_pick: "genmarshal.patch"
 # https://github.com/NixOS/nixpkgs/blob/257aff801079890e2c5ed3fa5b8e5f818f611fad/pkgs/development/tools/build-managers/meson/fix-rpath.patch
 cherry_pick: "rpath.patch"
 
+cherry_pick: "launcher.patch"
+
 pythonbuilder: {}
 
 runtime_dep: "ninja"

pkgs/meson/launcher.patch

@@ -0,0 +1,11 @@
+--- old/mesonbuild/mesonmain.py	2020-05-02 22:37:55.035436359 +0200
++++ new/mesonbuild/mesonmain.py	2020-05-02 22:38:28.588262968 +0200
+@@ -225,6 +225,7 @@
+         launcher = sys.executable
+     else:
+         launcher = os.path.realpath(sys.argv[0])
++    launcher = '/ro/meson-amd64-0.54.1-7/bin/meson'
+     return run(sys.argv[1:], launcher)
+
+ if __name__ == '__main__':
+Binary files /dev/null and new/mesonbuild/mesonmain.py~ differ