winbuild: Free name $(CC) in Makefile (#950)

In the old line number 290, CC and CURL_CC had the same value. After
that, /DCURL_STATICLIB was added to CC but not CURL_CC (intended?).

This gets rid of the CC variable entirely. It is a first step to make it
possible to manualyl set a CC variable in order to be able to change the
compiler.