Verify minimal clang version in cmake.

cmake/EthCompilerSettings.cmake

@@ -65,11 +65,8 @@ if (("${CMAKE_CXX_COMPILER_ID}" MATCHES "GNU") OR ("${CMAKE_CXX_COMPILER_ID}" MA
 
 	# Additional GCC-specific compiler settings.
 	if ("${CMAKE_CXX_COMPILER_ID}" MATCHES "GNU")
-
 		# Check that we've got GCC 8.0 or newer.
-		execute_process(
-			COMMAND ${CMAKE_CXX_COMPILER} -dumpversion OUTPUT_VARIABLE GCC_VERSION)
-		if (NOT (GCC_VERSION VERSION_GREATER 8.0 OR GCC_VERSION VERSION_EQUAL 8.0))
+		if (NOT (CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL 8.0))
 			message(FATAL_ERROR "${PROJECT_NAME} requires g++ 8.0 or greater.")
 		endif ()
 
@@ -78,6 +75,11 @@ if (("${CMAKE_CXX_COMPILER_ID}" MATCHES "GNU") OR ("${CMAKE_CXX_COMPILER_ID}" MA
 
 	# Additional Clang-specific compiler settings.
 	elseif ("${CMAKE_CXX_COMPILER_ID}" MATCHES "Clang")
+		# Check that we've got clang 7.0 or newer.
+		if (NOT (CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL 7.0))
+			message(FATAL_ERROR "${PROJECT_NAME} requires clang++ 7.0 or greater.")
+		endif ()
+
 		if ("${CMAKE_SYSTEM_NAME}" MATCHES "Darwin")
 			# Set stack size to 32MB - by default Apple's clang defines a stack size of 8MB.
 			# Normally 16MB is enough to run all tests, but it will exceed the stack, if -DSANITIZE=address is used.