CMake: Don't compile PRC's encryptStream unless we have OpenSSL.

dtool/src/prc/CMakeLists.txt

@@ -53,7 +53,6 @@ set(P3PRC_SOURCES
   configVariableManager.cxx
   configVariableSearchPath.cxx
   configVariableString.cxx
-  encryptStreamBuf.cxx encryptStream.cxx
   nativeNumericData.cxx
   notify.cxx
   notifyCategory.cxx
@@ -70,6 +69,10 @@ if(ANDROID)
     androidLogStream.cxx)
 endif()
 
+if(HAVE_OPENSSL)
+  list(APPEND P3PRC_SOURCES encryptStreamBuf.cxx encryptStream.cxx)
+endif()
+
 set(P3PRC_IGATEEXT
   streamReader_ext.cxx
   streamReader_ext.h