Configure: Improve the d_prctl_set_name detection for Android

On some versions of Android (e.g., 4.2.2, API level 17)
the header that defines the constants used for this test
will not work if unistd.h isn't already included.
Android hints: define d_procselfexe and procselfexe if needed

Only affects cross-compile builds; currently the tests
in Configure for this feature will be run on the host,
not the target system, which makes the detection of the
feature unreliable.

Since we know that /proc/self/exe exists, just define it
and set $procselfexe to that.
Android hints: Always append /system/lib and /vendor/lib in libpth

These are the canonical directories for libraries on Android.
This is only mildly important for native builds, since
whatever toolchain was installed will likely provide
their own /lib, but is quite important for cross builds.