Add compiler help to OpenCL.make

OpenCL.make

@@ -1,15 +1,22 @@
 
 ifndef COMPILER
-$(info Define a compiler to set common defaults, i.e make COMPILER=GNU)
+define compiler_help
+Set COMPILER to ensure correct flags are set.
+Available compilers are:
+  GNU CLANG INTEL CRAY
+endef
+$(info $(compiler_help))
 endif
 
 COMPILER_ = $(CXX)
 COMPILER_GNU = g++
+COMPILER_CLANG = clang++
 COMPILER_INTEL = icpc
 COMPILER_CRAY = CC
 
 FLAGS_ = -O3 -std=c++11
 FLAGS_GNU = -O3 -std=c++11
+FLAGS_CLANG = -O3 -std=c++11
 FLAGS_INTEL = -O3 -std=c++11
 FLAGS_CRAY = -O3 -hstd=c++11
 CXXFLAGS=$(FLAGS_$(COMPILER))