Don't install header files unless told so

Change-Id: Icebca00ad1f179c0fa2d15c38e6cec2e85babb76
Reviewed-on: http://review.couchbase.org/32433
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>

CMakeLists.txt

@@ -25,7 +25,9 @@ TARGET_LINK_LIBRARIES(conflate ${CURL_LIBRARIES} platform ${ZLIB})
 SET_TARGET_PROPERTIES(conflate PROPERTIES SOVERSION 1.0.0)
 SET_TARGET_PROPERTIES(conflate PROPERTIES COMPILE_FLAGS -DBUILDING_LIBCONFLATE=1)
 
+IF (INSTALL_HEADER_FILES)
 INSTALL (FILES include/libconflate/conflate.h DESTINATION include/libconflate)
+ENDIF (INSTALL_HEADER_FILES)
 
 INSTALL(TARGETS conflate
   RUNTIME DESTINATION bin