configure: fix PKG_CHECK_MODULES invocation

Patch by chutz.
https://trac.xiph.org/ticket/1825
build: Use pkgconf macros to check for ogg presence

And make the selection not automagic.

Unbreaks cross-building w/out ogg.

Signed-off-by: Tristan Matthews <le.businessman@gmail.com>
whitespace: drop trailing spaces