recipe/build.sh: force Qt to honor our build enviroment variables
recipe/build.sh: force Qt to honor our build enviroment variables

Now that we're really supporting multiple build toolchains, it is important
for build systems to honor standard environment variables like $CC and
$CXXFLAGS. Qt's system doesn't do this consistently. Here we patch the build
script to force it to do so.

conda-forge/qt-feedstock,conda-forge/qt-feedstock