Check for speex_encoder_init instead of speex_encode to determine the presense of libspeex.
When compiled with no floating numbers support it does provide speex_encode_int and no speex_encode.
speex_encoder_init is available regardless of "float vs. int"-configuration.
--- configure
+++ configure
@@ -27881,7 +27881,7 @@
          pbxlibdir="-L${SPEEX_DIR}"
       fi
    fi
-   pbxfuncname="speex_encode"
+   pbxfuncname="speex_encoder_init"
    if test "x${pbxfuncname}" = "x" ; then   # empty lib, assume only headers
       AST_SPEEX_FOUND=yes
    else