Adds explanation on how to fix cgreen meson build.

subprojects/packagefiles/cgreen/meson.build

@@ -1,3 +1,30 @@
+# Most of the setup is copy&paste, how to reproduce (the hard way..):
+# 0. try to use `cgreen` as normal `cmake` subproject
+# 1. add bare git-wrap for cgreen -> `subprojects/cgreen.wrap`:
+#    > [wrap-git]
+#    > url = https://github.com/cgreen-devs/cgreen.git
+#    > revision = master
+#    > depth = 1
+# 2. in `<project_root>/meson.build`, add normal cmake subproject setup:
+#    > cmake = import('cmake')
+#    > cgreen = cmake.subproject('cgreen')
+#    > # you may want to lookup available targets before
+#    > message('CMaket targets:\n - ' + '\n - '.join(cgreen.target_list()))
+#    > cgreen_dep = cgreen.dependency('cgreen_shared')
+#    > cgreen_runner_dep = cgreen.dependency('cgreen_runner')
+# 3. compile with meson (`meson compile -C <builddir>`)
+# 4. look up generated meson.build in `<builddir>/subprojects/cgreen/meson.build`
+# 5. copy & paste into subprojects/packagefiles
+#    a. add meson overrides for wanted dependencies:
+#       > meson.override_dependency('cgreen', cgreen_shared_dep)
+#       > meson.override_find_program('cgreen-runner', cgreen_runner)
+#    b. remove all other generated targets
+# 6. fix the errors you faced before :)
+#    a. remove redundant `FILENAME` defines
+#    b. checkout how `config.h` is generated (see `subprojects/cgreen/config.h.cmake`)
+#    c. apply that to [meson setup](https://mesonbuild.com/Configuration.html)
+#       or create a minimal working substitute ;) -> see below (:30-35)
+
 project('cgreen', 'c', 'cpp')
 
 configure_file(