windows: tweak Makefile.mingw for easier usage

Default CC to `gcc` instead of `cc` because at least on the mingw
installation I had there was no `cc` executable. This avoids having
to type `CC=gcc` in all the `make` commands.

Also append to `CFLAGS` instead of defining if not previously defined.
These flags are required to build libuv, so they should not be
overridden if other extra CFLAGS are supplied via the command
line as well.

Makefile.mingw

@@ -12,14 +12,16 @@
 # ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
 # OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 
-CFLAGS = -Wall \
-         -Wextra \
-         -Wno-unused-parameter \
-         -Iinclude \
-         -Isrc \
-         -Isrc/win \
-         -DWIN32_LEAN_AND_MEAN \
-         -D_WIN32_WINNT=0x0600
+CC = gcc
+
+CFLAGS += -Wall \
+          -Wextra \
+          -Wno-unused-parameter \
+          -Iinclude \
+          -Isrc \
+          -Isrc/win \
+          -DWIN32_LEAN_AND_MEAN \
+          -D_WIN32_WINNT=0x0600
 
 INCLUDES = include/stdint-msvc2008.h \
            include/tree.h \