This is needed to move our modified SDL sources under external/qemu/distrib.
As per joeo's request, this change also enables target Objective-C compilation, letting it error at build time.
Until now only generated assembly or C++ files would be compiled. This patch extends the build
system to compile generated C files as well. The new rule is modeled on the existing rules for
compiling generated C++ files and the existing rule for compiling ordinary C files.