Android: fix microperl build parameters

Unlike miniperl, microperl doesnt include unistd.h

add required definitions of execl() and _exit()

update android configure script

override some config vars like android wchar functions
update android config, use some build vars

remove absolute paths in variables, use $ANDROID_BUILD_TOP as
reference.

TODO: remove temporary SYSROOT

remove widechar func defines, bionic implementation is incorrect