tools: add benchmark for crypto functions.

The benchmark tests the speed of the following algorithms:
MD5, SHA-1, SHA-256, SHA-512, RIPEMD-160, AES-128.

It can optionally be built to perform the same benchmark on
other crypto libraries, for comparison purposes.
The supported libraries are:
- crypto:   OpenSSL's libcrypto;
- gcrypt:   GnuTLS's libgcrypt;
- tomcrypt: LibTomCrypt
To enable them, use this syntax:
make VERSUS=crypto+gcrypt+tomcrypt tools/crypto_bench
They do not need to have been enabled in configure.

.gitignore

@@ -63,6 +63,7 @@
 /tools/aviocat
 /tools/ffbisect
 /tools/bisect.need
+/tools/crypto_bench
 /tools/cws2fws
 /tools/fourcc2pixfmt
 /tools/ffescape