Use openssl binary of no base64 binary is available

Suggested by maxfieb (#21)

CMakeLists.txt

 find_program(HELP2MAN help2man DOC "path to help2man executable")
 find_program(DD dd DOC "path to dd executable")
 find_program(BASE64 base64 DOC "path to base64 executable")
+find_program(OPENSSL openssl DOC "path to OpenSSL executable")
 find_program(INSSERV insserv DOC "path to insserv executable")
 find_program(CHKCONFIG chkconfig DOC "path to chkconfig executable")
 find_library(LIBSOCKET socket)
 find_library(LIBNSL nsl)
 
+if(BASE64)
+    set(BASE64_ENCODE "${BASE64}")
+elseif(OPENSSL)
+    set(BASE64_ENCODE "${OPENSSL} base64 -e")
+else()
+    set(BASE64_ENCODE "")
+endif()
+
 check_include_file(sys/wait.h HAVE_SYS_WAIT_H)
 if(HAVE_SYS_WAIT_H)
     add_definitions(-DHAVE_SYS_WAIT_H)

postinstall.cmake.in

 set(INIT_FLAVOR "@INIT_FLAVOR@")
 set(SECRET_FILE "@PROJECT_NAME@.secret")
 set(DD "@DD@")
-set(BASE64 "@BASE64@")
+set(BASE64_ENCODE "@BASE64_ENCODE@")
 set(INSSERV "@INSSERV@")
 set(CHKCONFIG "@CHKCONFIG@")
 
 	file(INSTALL FILES "@CMAKE_CURRENT_BINARY_DIR@/@PROJECT_NAME@.systemd" DESTINATION "${SYSCONF_DIR}/systemd/system" RENAME "@PROJECT_NAME@.service")
 endif()
 
-if(GENERATE_SRS_SECRET AND DD AND BASE64 AND EXISTS "/dev/urandom" AND NOT EXISTS "$ENV{DESTDIR}${SYSCONF_DIR}/${SECRET_FILE}")
+if(GENERATE_SRS_SECRET AND DD AND BASE64_ENCODE AND EXISTS "/dev/urandom" AND NOT EXISTS "$ENV{DESTDIR}${SYSCONF_DIR}/${SECRET_FILE}")
 	message(STATUS "Generating secret key")
 	execute_process(
 		COMMAND ${DD} if=/dev/urandom bs=18 count=1
-		COMMAND ${BASE64}
+                COMMAND ${BASE64_ENCODE}
 		OUTPUT_FILE "@CMAKE_CURRENT_BINARY_DIR@/${SECRET_FILE}"
 		ERROR_QUIET
 		OUTPUT_STRIP_TRAILING_WHITESPACE