Clarify YAML_CPP_SUPPORT_MERGE_KEYS CMake option usage

It doesn't need to be defined when building the library, since all the
related code is in its header files.

Author: Ortham

CMakeLists.txt

@@ -41,7 +41,7 @@ enable_testing()
 option(YAML_CPP_BUILD_TESTS "Enable testing" ON)
 option(YAML_CPP_BUILD_TOOLS "Enable parse tools" ON)
 option(YAML_CPP_BUILD_CONTRIB "Enable contrib stuff in library" ON)
-option(YAML_CPP_SUPPORT_MERGE_KEYS "Build with support for YAML merge keys ('<<'). Also requires '#define YAML_CPP_SUPPORT_MERGE_KEYS' when linking." OFF)
+option(YAML_CPP_SUPPORT_MERGE_KEYS "Support YAML merge keys ('<<') in yaml-cpp's executable targets. Use '#define YAML_CPP_SUPPORT_MERGE_KEYS' instead when linking from another project." OFF)
 
 ## Build options
 # --> General