Append CXX to the list of needed languages

The replxx is written in C++ and it is enabled by default. We need to
specify this to correctly pass user specific CXXFLAGS.

CMakeLists.txt

@@ -21,7 +21,7 @@ ENDIF()
 
 SET(RSPAMD_VERSION         "${RSPAMD_VERSION_MAJOR}.${RSPAMD_VERSION_MINOR}")
 
-PROJECT(rspamd VERSION "${RSPAMD_VERSION}" LANGUAGES C ASM)
+PROJECT(rspamd VERSION "${RSPAMD_VERSION}" LANGUAGES C CXX ASM)
 
 # This is supported merely with cmake 3.1
 SET(CMAKE_C_STANDARD 11)