fftw: Check for TOOLCHAIN_OPTIONS to be non-empty before sed ops

Ensures that we do not apply empty sed expression which might happen
when building native recipe

Cc: jan vermaete <jan.vermaete@gmail.com>
Signed-off-by: Khem Raj <raj.khem@gmail.com>

meta-oe/recipes-support/fftw/fftw_3.3.10.bb

@@ -55,7 +55,7 @@ do_configure() {
 do_compile() {
     for lib in fftw fftwl fftwf; do
         cd ${WORKDIR}/build-$lib
-        sed -i -e 's|${TOOLCHAIN_OPTIONS}||g' config.h
+        test -n "${TOOLCHAIN_OPTIONS}" && sed -i -e 's|${TOOLCHAIN_OPTIONS}||g' config.h
         autotools_do_compile
     done
 }