FTI cannot be built without OpenSSL

[christoph-conrads]

During configuration, CMake announces falling back to the MD5 algorithm when the OpenSSL headers cannot be found:

$ cmake -DCMAKE_INSTALL_PREFIX=/tmp/prefix.fti -- ~/fti
-- The C compiler identification is GNU 7.5.0
-- Check for working C compiler: /usr/bin/cc
-- Check for working C compiler: /usr/bin/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Found ZLIB: /usr/lib/x86_64-linux-gnu/libz.so (found version "1.2.11") 
-- Found MPI: /usr/bin/mpicc
      
-- Could NOT find OpenSSL, try to
   set the path to OpenSSL root folder in the system variable OPENSSL_ROOT_DIR (missing:  OPENSSL_SSL_LIBRARY OPENSSL_CRYPTO_LIBRARY OPENSSL_INCLUDE_DIR)
         
-- Using slower build in MD5 algorithm.
[snip]

Nevertheless, an OpenSSL header is still used:

[ 86%] Building C object testing/suites/features/differentialCkpt/CMakeFiles/diff_test.exe.dir/diff_test_func.c.o
In file included from /home/christoph/fti/testing/suites/features/differentialCkpt/diff_test_func.c:7:0:
/home/christoph/fti/testing/suites/features/differentialCkpt/diff_test.h:18:13: fatal error: openssl/md5.h: No such file or directory
 #   include <openssl/md5.h>
             ^~~~~~~~~~~~~~~

[christoph-conrads]

accidental double post in #348