configure does not find speex when using non-standard prefix

I tried to compile icecast 2.4.0 using a non-standard prefix as follows:

./configure --prefix=<mydir>

The libraries libogg, libvorbis and libtheora are found, but libspeex is not:

checking for libogg... ok
checking for libvorbis... ok
checking for libtheora... ok
checking for libspeex... configure: WARNING: Speex support disabled!

The reason for this is that the configure script does not append SPEEX_CFLAGS to CPPFLAGS. Attached patch contains a fix for m4/speex.m4 that temporarily modifies CPPFLAGS in a way similar to the other libraries.