Pass CC explicitly to FFTW configure to suppress cross-tools warning

Without CC=, autoconf issues "using cross tools not prefixed with host
triplet" because it cannot match the detected compiler against --host.
Passing CC=$(MINGW_PREFIX)-gcc directly resolves the ambiguity.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Makefile.windows

@@ -40,6 +40,7 @@ FFTW_BUILD_F := $(FFTW_INST)/build-float
 
 FFTW_CONF_COMMON = \
   --host=$(MINGW_PREFIX) \
+  CC=$(CC) \
   --prefix=$(FFTW_INST) \
   --enable-shared \
   --disable-static \