CMake: Add compiler warning about implicit fallthough

cmake/EthCheckCXXCompilerFlag.cmake

@@ -0,0 +1,23 @@
+include(CheckCXXCompilerFlag)
+
+# Adds CXX compiler flag if the flag is supported by the compiler.
+#
+# This is effectively a combination of CMake's check_cxx_compiler_flag()
+# and add_compile_options():
+#
+#    if(check_cxx_compiler_flag(flag))
+#        add_compile_options(flag)
+#
+function(eth_add_cxx_compiler_flag_if_supported FLAG)
+  # Remove leading - or / from the flag name.
+  string(REGEX REPLACE "^-|/" "" name ${FLAG})
+  check_cxx_compiler_flag(${FLAG} ${name})
+  if(${name})
+    add_compile_options(${FLAG})
+  endif()
+
+  # If the optional argument passed, store the result there.
+  if(ARGV1)
+    set(${ARGV1} ${name} PARENT_SCOPE)
+  endif()
+endfunction()

cmake/EthCompilerSettings.cmake

@@ -4,7 +4,7 @@
 # CMake file for cpp-ethereum project which specifies our compiler settings
 # for each supported platform and build configuration.
 #
-# See http://www.ethdocs.org/en/latest/ethereum-clients/cpp-ethereum/.
+# The documentation for cpp-ethereum is hosted at http://cpp-ethereum.org
 #
 # Copyright (c) 2014-2016 cpp-ethereum contributors.
 #------------------------------------------------------------------------------
@@ -14,18 +14,15 @@
 #
 # These settings then end up spanning all POSIX platforms (Linux, OS X, BSD, etc)
 
-include(CheckCXXCompilerFlag)
+include(EthCheckCXXCompilerFlag)
 
-check_cxx_compiler_flag(-fstack-protector-strong have_stack_protector_strong)
-if (have_stack_protector_strong)
-	add_compile_options(-fstack-protector-strong)
-else()
-	check_cxx_compiler_flag(-fstack-protector have_stack_protector)
-	if(have_stack_protector)
-		add_compile_options(-fstack-protector)
-	endif()
+eth_add_cxx_compiler_flag_if_supported(-fstack-protector-strong have_stack_protector_strong_support)
+if(NOT have_stack_protector_strong_support)
+	eth_add_cxx_compiler_flag_if_supported(-fstack-protector)
 endif()
 
+eth_add_cxx_compiler_flag_if_supported(-Wimplicit-fallthrough)
+
 if (("${CMAKE_CXX_COMPILER_ID}" MATCHES "GNU") OR ("${CMAKE_CXX_COMPILER_ID}" MATCHES "Clang"))
 
 	# Use ISO C++11 standard language.
@@ -83,12 +80,6 @@ if (("${CMAKE_CXX_COMPILER_ID}" MATCHES "GNU") OR ("${CMAKE_CXX_COMPILER_ID}" MA
 			message(FATAL_ERROR "${PROJECT_NAME} requires g++ 4.7 or greater.")
 		endif ()
 
-		# Until https://github.com/ethereum/solidity/issues/2479 is handled
-		# disable all implicit fallthrough warnings in the codebase for GCC > 7.0
-		if (CMAKE_CXX_COMPILER_VERSION VERSION_GREATER 7.0)
-			add_compile_options(-Wno-implicit-fallthrough)
-		endif()
-
 	# Additional Clang-specific compiler settings.
 	elseif ("${CMAKE_CXX_COMPILER_ID}" MATCHES "Clang")