Also set CONFIGURE.build to null since the configure script has no clue about --build.
X264.CONFIGURE.extra += --disable-lavf --disable-ffms --disable-avs --disable-swscale --disable-gpac --disable-lsmash --disable-cli
-ifeq (1-mingw,$(BUILD.cross)-$(BUILD.system))
+ifeq (1,$(BUILD.cross))
+ X264.CONFIGURE.build =
X264.CONFIGURE.extra += --cross-prefix=$(BUILD.spec)-
X264.CONFIGURE.env.CFLAGS = CFLAGS="-I$(call fn.ABSOLUTE,$(CONTRIB.build/)include) $(call fn.ARGS,X264.GCC,*archs *sysroot *minver ?extra)"
X264.CONFIGURE.env.LDFLAGS = LDFLAGS="-L$(call fn.ABSOLUTE,$(CONTRIB.build/)lib) $(call fn.ARGS,X264.GCC,*archs *sysroot *minver)"