Use name in GCC_CLASSIC as C compiler for coverage testing, if set.

Apple removed support for gcov in the default gcc compiler chain,
when they moved to llvm.  This can be circumvented in XCode 4.2 by
using the gcc chain with gcc-4.2.  This patch allows setting
GCC_CLASSIC to the name of a real gcc executable (e.g. "gcc-4.2")
to allow coverage testing.

configure

@@ -416,6 +416,9 @@ fi
 
 if test $cover -eq 1; then
   CFLAGS="${CFLAGS} -fprofile-arcs -ftest-coverage"
+  if test -n "$GCC_CLASSIC"; then
+    CC=$GCC_CLASSIC
+  fi
 fi
 
 echo >> configure.log