Use $CC if variable is set

[RobinGeffroy]

Fixes commit e9a52aa
cc variable is set if CC is not set, and ignored otherwise.

I was unable to build zlib on Linux because $CC was set
To reproduce :

CC=gcc ./configure --shared
make

Results :

make
/usr/bin/ld: deflate.lo: relocation R_X86_64_PC32 against symbol `_length_code' can not be used when making a shared object; recompile with -fPIC
/usr/bin/ld: final link failed: bad value
collect2: error: ld returned 1 exit status
make[2]: *** [Makefile:282: libz.so.1.2.12] Error 1

[Mr-Clam]

More details in #646, which was closed without a fix.

[RobinGeffroy]

@Mr-Clam thanks for the link.
Since this is a regression caused by a commit for v1.2.12, I assumed it was unintentional.

[Mr-Clam]

Right. And to correct me comment, there is this commit in the develop branch: 05796d3

[RobinGeffroy]

Fixed in develop branch, closing this.