No longer force --enable-shared

According to GCC's documentation it is enabled by default since GCC >
2.95, see:
https://gcc.gnu.org/install/configure.html

doc/cross-gcc.sh

@@ -296,7 +296,7 @@ EOF
 		(cd "gcc-$TARGET" && "../gcc-$GCC_VERSION/configure" \
 			--target="$TARGET" --prefix="$PREFIX" \
 			--disable-nls --enable-languages=c,c++ \
-			--without-headers --enable-shared $GCC_FLAGS)
+			--without-headers $GCC_FLAGS)
 	fi
 	for target in $GCC_TARGETS; do
 		(cd "gcc-$TARGET" && $MAKE "$target")