Use "$(CC)" instead of "gcc" in Makefiles

To allow autoconf builds with CC=clang and legacy builds
(make -f makefile.orig linux-x86-64-clang) if no gcc is installed.

This also makes sure version checks etc use (gcc compatible) clang
instead of gcc for clang builds:

$ gcc -dumpversion
4.8.2

$ clang -dumpversion
4.2.1

$ gcc -dumpmachine
x86_64-redhat-linux

$ clang -dumpmachine
x86_64-redhat-linux-gnu

As a result, o5logon got slower for clang builds.

Before:
$ ../run/john --test --format=o5logon
Benchmarking: o5logon, Oracle O5LOGON protocol [SHA1 AES 32/64 AES-NI]... DONE
Raw:	4929K c/s real, 4929K c/s virtual

After:
$ ../run/john --test --format=o5logon
Benchmarking: o5logon, Oracle O5LOGON protocol [SHA1 AES 32/64 AES-oSSL]... DONE
Raw:	2655K c/s real, 2655K c/s virtual

(64 bit Linux)

Legacy builds (make -s -f Makefile.orig linux-x86-64-clang) still have another issue:
$ ../run/john --test=0 --format=keepass
Testing: KeePass [SHA256 AES 32/64 OpenSSL]... AES_set_decrypt_key failed in crypt!
AES_set_decrypt_key failed in crypt!
AES_set_decrypt_key failed in crypt!
AES_set_decrypt_key failed in crypt!
AES_set_decrypt_key failed in crypt!
AES_set_decrypt_key failed in crypt!
AES_set_decrypt_key failed in crypt!
AES_set_decrypt_key failed in crypt!
PASS

$ ../run/john --test=1 --format=keepass
Benchmarking: KeePass [SHA256 AES 32/64 OpenSSL]... AES_set_decrypt_key failed in crypt!
AES_set_decrypt_key failed in crypt!
AES_set_decrypt_key failed in crypt!
...
AES_set_decrypt_key failed in crypt!
DONE
Raw:	198 c/s real, 202 c/s virtual

The "AES_set_decrypt_key failed in crypt!" errors only occur in
about 50% of the test runs.

The 198 c/s is still much faster than the AES 32/64 OpenSSL speed of an
autoconfig clang build.

$ make -s distclean ; CC=clang ./configure && make -s -j 8
...
$ ../run/john --test --format=keepass
Benchmarking: KeePass [SHA256 AES 32/64 OpenSSL]... DONE
Raw:	126 c/s real, 126 c/s virtual