Patch status: here forever(?), not respecting flags is intentional

Original gen.py respects *FLAGS to some extend, but Qt blocked
this after a user reported a bug with overzealous *FLAGS[2].

So re-add, but also order later on to allow overriding flags.

Also add a way to pass flags to ninja used to build Gn.

TODO: fix cross
  Technically should use BUILD_CXXFLAGS and so here but, even
  without trying, cross is most likely broken given it insists
  on finding Gn in CBUILD rather than cross-compile it. To avoid
  more patching, an option may be BDEPEND="dev-qt/qtwebengine-gn"
  that would do INSTALL_GN=ON and not build qtwebengine.

Note that qtwebengine's Gn is modified, so dev-util/gn may not
be a straight forward to use as an alternative and be fragile.

[1] https://github.com/qt/qtwebengine-chromium/commit/19e8b974c8348dd6ba33153facc0b67a7ab774e1
[2] https://bugreports.qt.io/browse/QTBUG-64759
[3] https://bugs.gentoo.org/652172
[4] https://bugs.gentoo.org/920758
--- a/src/3rdparty/gn/build/gen.py
+++ b/src/3rdparty/gn/build/gen.py
@@ -1014,4 +1014,7 @@
   executables['gn_unittests']['libs'].extend(static_libraries.keys())
 
+  cflags += os.environ.get('CXXFLAGS', '').split()
+  cflags += os.environ.get('CPPFLAGS', '').split()
+  ldflags += os.environ.get('LDFLAGS', '').split()
   WriteGenericNinja(path, static_libraries, executables, cxx, ar, ld,
                     platform, host, options, args_list,
--- a/src/gn/CMakeLists.txt
+++ b/src/gn/CMakeLists.txt
@@ -40,4 +40,5 @@
 endif()
 
+separate_arguments(NINJAFLAGS UNIX_COMMAND "$ENV{NINJAFLAGS}")
 file(MAKE_DIRECTORY ${GN_BINARY_DIR})
 
@@ -75,5 +76,5 @@
         $<$<AND:$<PLATFORM_ID:Darwin>,$<BOOL:${OSX_ARCH}>>:--osx-architectures>
         $<$<AND:$<PLATFORM_ID:Darwin>,$<BOOL:${OSX_ARCH}>>:${OSX_ARCH}>
-    COMMAND Ninja::ninja -C ${GN_BINARY_DIR}/$<CONFIG> ${GN_EXECUTABLE}
+    COMMAND Ninja::ninja ${NINJAFLAGS} -C ${GN_BINARY_DIR}/$<CONFIG> ${GN_EXECUTABLE}
     VERBATIM
     USES_TERMINAL