check if libdl and libz are really needed

[pabuhler]

    In this case |I found that it was mileading mesage. If you will look closet on the bottom openssl detection

https://github.com/cisco/libsrtp/blob/cc362ae5b3d30c89d8dc00c22f84f615fb02d956/configure.ac#L208-L256 you can fiund libdl and libz detections. I had no installed libz devel resources and that message was printed by incorrecvt logic in tjhat section Looks like libz detection can be removed because nothing in source tree is usimg libz.

[tkloczko@devel-g2v libsrtp-2.4.2]$ grep zlib.h -r
[tkloczko@devel-g2v libsrtp-2.4.2]$

Whole section could be dramatically reduced if PKG_CHECK_MODULES() aclocal macro would be used.

Originally posted by @kloczek in https://github.com/cisco/libsrtp/issues/599#issuecomment-1338268618

[pabuhler]

Follow up on @kloczek 's comments in #599