curl: Fix test for PPC

The tests for PowerPC in `3rdparty/curl-7.19.7/include/curl/curlbuild.h` use preprocessor symbols (`__ppc__` and `__ppc64__`) which are obsolete.  This results in a FTBFS.

The symbols defined by current gcc are `__powerpc__` and `__powerpc64__`.  Note that both are defined on a 64-bit system.