configure: Use require_pkg_config for Speex

Distributors and integrators nowadays have less problems by leveraging
pkg-config files than having to set custom CFLAGS and LDFLAGS.

configure

@@ -4118,7 +4118,7 @@ enabled libopus           && require_pkg_config opus opus_multistream.h opus_mul
 enabled libpulse          && require_pkg_config libpulse-simple pulse/simple.h pa_simple_new
 enabled librtmp           && require_pkg_config librtmp librtmp/rtmp.h RTMP_Socket
 enabled libschroedinger   && require_pkg_config schroedinger-1.0 schroedinger/schro.h schro_init
-enabled libspeex          && require libspeex speex/speex.h speex_decoder_init -lspeex
+enabled libspeex          && require_pkg_config speex speex/speex.h speex_decoder_init -lspeex
 enabled libtheora         && require libtheora theora/theoraenc.h th_info_init -ltheoraenc -ltheoradec -logg
 enabled libtwolame        && require libtwolame twolame.h twolame_init -ltwolame
 enabled libvo_aacenc      && require libvo_aacenc vo-aacenc/voAAC.h voGetAACEncAPI -lvo-aacenc