[libclc] Clarify option help message

This should make it more obvious it applies only to libclc.

Author: frasercrmck

libclc/CMakeLists.txt

@@ -32,7 +32,7 @@ set_property(DIRECTORY APPEND PROPERTY CMAKE_CONFIGURE_DEPENDS
 set( LIBCLC_MIN_LLVM 3.9.0 )
 
 set( LIBCLC_TARGETS_TO_BUILD "all"
-    CACHE STRING "Semicolon-separated list of targets to build, or 'all'." )
+    CACHE STRING "Semicolon-separated list of libclc targets to build, or 'all'." )
 
 option( ENABLE_RUNTIME_SUBNORMAL "Enable runtime linking of subnormal support." OFF )