Revert 9518ab65: Allow user to specify CC env var (CC=clang make)

Conditional assignment (assign if not set) to CC and AR never works
because their default values with GNU make are `cc` and `ar` respectively.
Build with `make AR=your-ar CC=your-cc` if you want to override them.

Fixes #89.

config-mingw.mk

@@ -20,8 +20,8 @@
 
 # Use make -f Makefile.gcc PREFIX=i686-w64-mingw32-
 # for cross compilation
-CC ?= $(PREFIX)gcc
-AR ?= $(PREFIX)ar
+CC = $(PREFIX)gcc
+AR = $(PREFIX)ar
 E=.exe
 
 CFLAGS=$(CPPFLAGS) -g --std=gnu89 -D_WIN32_WINNT=0x0501 -Isrc/ares/config_win32

config-unix.mk

@@ -18,8 +18,8 @@
 # FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
 # IN THE SOFTWARE.
 
-CC ?= $(PREFIX)gcc
-AR ?= $(PREFIX)ar
+CC = $(PREFIX)gcc
+AR = $(PREFIX)ar
 E=
 CSTDFLAG=--std=c89 -pedantic
 CFLAGS=-g