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
Merge pull request #625 from frank-dittrich/bleeding-jumbo

Use "$(CC)" instead of "gcc" in Makefiles
some more Makefile changes

for make clean, use the parent's jobserver for subdirectories
redirect some unwanted possible stderr output to /dev/null
Merge pull request #626 from frank-dittrich/bleeding-jumbo

some more Makefile changes
Better Kerberos 5 detection. Detect MKShim and Heimdal.
Add krb5support to list of libs to search for Kerberos 5 functions.
Revert last patch and simply disable krb5-18/23 formats if Heimdal
was found (until we know, and handle, the differences).
Fixes for Heimdal Kerberos.
Fixes for broken krb5 systems, like cygwin. Removed the  not HEIMDAL defines.  Changed ifdefs to #ifs for some HAVE_XYZ defines.  WE should do that across the board
added lbz2 to gpg2john. Code was there, just enabled it in autoconf