Support for GCC has been dropped from NDK r13.  Let's switch
everything to clang before they really remove GCC completely.

Causes compatibility problems, because FFmpeg expects the GNU as
syntax.

This FFmpeg bug report has some information, but it was closed as
"invalid": https://trac.ffmpeg.org/ticket/5158

DESTDIR is for installing to a chroot, but this is not one.  Instead,
we want real absolute paths in the pkg-config file.

Fixes various static library linker failures ("bad extended name entry
at header"), because the wrong "ranlib" was used.

autoconf depends on libtoolize.