Building for PASE from AIX

• Pass -D__PASE__ to your CPPFLAGS to disable some stuff not supported by OS/400, and work around bugs in V7R1, the earliest release we support. When building from OS/400 with the IBM supplied toolchain installed from their Yum repo, this flag will be passed automatically.

• Pass --enable-minimal=shared_perfcounters to disable shared performance counters, as OS/400 doesn't support the POSIX SHM APIs; only SysV. This disablement doesn't matter too much, as the iOS builds from MS also use this option.