We want compiler check arguments (-O0, -fpermissive, etc) to override
all other arguments, and we want extra_args passed in by the build file
to always override everything.
To do this properly, we must split include arguments out, append them
first, append all other arguments as usual, and then append the rest.
As part of this, we also add the compiler check flags to the
cc.compiles() and cc.links() helper functions since they also most
likely need them.
Also includes a unit test for all this.
This can only be done as a unit test because it requires changes to the
environment. This also means we need to pass the current environment to
subprocess.check_output, which we should always do anyway since the
environment is torn down and restored between each test.
Nirbheek Chauhan
Jussi Pakkanen
Elliott Sales de Andrade