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
os_support: fix misleading comments

cherry-picked from speexdsp 86779a06f6500d041573d6252d4971d3bfcb4b18
Merge remote-tracking branch 'upstream/master'
speexenc: don't read past the end of WAV data

Fixes https://trac.xiph.org/ticket/2036
Reported by justcar
Merge remote-tracking branch 'upstream/master'