Stop setting CMAKE_CXX_FLAGS globaly

[D4N]

We are currently adding a bunch of additional compiler flags to detect issues. However, we are setting these globally via set(CMAKE_CXX_FLAGS ...), which isn't too great. Instead we should add the flags only to our targets via target_compile_options. This would be beneficial as we could enable -fsanitize=fuzzer only for the fuzzing targets in fuzz/.

[clanmills]

I think this is a historical issue and can be closed.

646 rmills@rmillsm1:~/gnu/github/exiv2/main $ find . -name "*.cmake" -o -name CMakeLists.txt | xargs grep CXX | grep -v build
./CMakeLists.txt:    LANGUAGES CXX
./cmake/compilerFlagsExiv2.cmake:include(CheckCXXCompilerFlag)
./cmake/compilerFlagsExiv2.cmake:            if ( CMAKE_CXX_COMPILER_VERSION VERSION_GREATER 4.0 )
./cmake/compilerFlagsExiv2.cmake:            if ( CMAKE_CXX_COMPILER_VERSION VERSION_GREATER 5.0 )
./cmake/compilerFlagsExiv2.cmake:            if ( CMAKE_CXX_COMPILER_VERSION VERSION_GREATER 6.0 )
./cmake/compilerFlagsExiv2.cmake:            if ( CMAKE_CXX_COMPILER_VERSION VERSION_GREATER 7.0 )
./cmake/compilerFlagsExiv2.cmake:            if ( CMAKE_CXX_COMPILER_VERSION VERSION_GREATER 3.4.2 )
./cmake/compilerFlagsExiv2.cmake:            if ( CMAKE_CXX_COMPILER_VERSION VERSION_GREATER 3.8.1 )
./cmake/compilerFlags.cmake:include(CheckCXXCompilerFlag)
./cmake/compilerFlags.cmake:    if (${CMAKE_CXX_COMPILER_ID} STREQUAL GNU)
./cmake/compilerFlags.cmake:    elseif (${CMAKE_CXX_COMPILER_ID} MATCHES "Clang")
./cmake/compilerFlags.cmake:    set (CMAKE_CXX_FLAGS_DEBUG "-g3 -gstrict-dwarf -O0")
./cmake/compilerFlags.cmake:            set(CMAKE_XCODE_ATTRIBUTE_CLANG_CXX_LIBRARY "libstdc++")
./cmake/compilerFlags.cmake:            set(CMAKE_XCODE_ATTRIBUTE_CLANG_CXX_LIBRARY "libc++")
./cmake/compilerFlags.cmake:        add_compile_options(-Wp,-D_GLIBCXX_ASSERTIONS)
./cmake/compilerFlags.cmake:                if ( CMAKE_CXX_COMPILER_VERSION VERSION_GREATER 4.9 )
./cmake/compilerFlags.cmake:                elseif( CMAKE_CXX_COMPILER_VERSION VERSION_GREATER 4.8 )
./cmake/compilerFlags.cmake:                if ( CMAKE_CXX_COMPILER_VERSION VERSION_GREATER 4.9 )
./cmake/compilerFlags.cmake:                elseif ( CMAKE_CXX_COMPILER_VERSION VERSION_GREATER 3.4 )
./cmake/compilerFlags.cmake:                elseif( CMAKE_CXX_COMPILER_VERSION VERSION_GREATER 3.1 )
./cmake/compilerFlags.cmake:                set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${SANITIZER_FLAGS}")
./cmake/compilerFlags.cmake:            set(CMAKE_CXX_COMPILER ${CLCACHE})
./cmake/compilerFlags.cmake:      CMAKE_CXX_FLAGS_DEBUG
./cmake/compilerFlags.cmake:      CMAKE_CXX_FLAGS_MINSIZEREL
./cmake/compilerFlags.cmake:      CMAKE_CXX_FLAGS_RELEASE
./cmake/compilerFlags.cmake:      CMAKE_CXX_FLAGS_RELWITHDEBINFO
./cmake/compilerFlags.cmake:        string(REGEX REPLACE "/W[0-4]" "/W4" CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}")
./cmake/mainSetup.cmake:if (NOT CMAKE_CXX_COMPILER_ID STREQUAL "AppleClang")
./cmake/mainSetup.cmake:    set(CMAKE_CXX_VISIBILITY_PRESET hidden)
./cmake/packaging.cmake:  if (${CMAKE_CXX_COMPILER_ID} MATCHES "Clang")
./cmake/printSummary.cmake:message( STATUS "Compiler info: ${CMAKE_CXX_COMPILER_ID} (${CMAKE_CXX_COMPILER}) ; version: ${CMAKE_CXX_COMPILER_VERSION}")
./cmake/printSummary.cmake:message( STATUS "CMAKE_CXX_STANDARD:${CMAKE_CXX_STANDARD}" )
./cmake/printSummary.cmake:message( STATUS "General:           ${CMAKE_CXX_FLAGS}" )
./cmake/printSummary.cmake:message( STATUS "Debug:             ${CMAKE_CXX_FLAGS_DEBUG}" )
./cmake/printSummary.cmake:message( STATUS "Release:           ${CMAKE_CXX_FLAGS_RELEASE}" )
./cmake/printSummary.cmake:message( STATUS "RelWithDebInfo:    ${CMAKE_CXX_FLAGS_RELWITHDEBINFO}" )
./cmake/printSummary.cmake:message( STATUS "MinSizeRel:        ${CMAKE_CXX_FLAGS_MINSIZEREL}" )
./cmake/generateConfigFile.cmake:include(CheckIncludeFileCXX)
./cmake/generateConfigFile.cmake:include(CheckCXXSourceCompiles)
./cmake/generateConfigFile.cmake:include(CheckCXXSymbolExists)
./cmake/FindIconv.cmake:  # If neither C nor CXX are loaded, implicit iconv makes no sense.
./src/CMakeLists.txt:if (${CMAKE_CXX_COMPILER_ID} STREQUAL GNU)
647 rmills@rmillsm1:~/gnu/github/exiv2/main $ 

Setting we almost never set CMAKE_CXX_FLAGS. There has been quite a number of changes involving CheckCXXCompilerFlag to enable the CMake code to set options which are known to be supported by the compiler. This is particularly relevant for setting options for ARM processors (Rasperry PI and Apple Silicon) and relatively obscure compiler options for matters such as stack protection. @piponazo has accepted an assignment to investigate some of those. For example: #1355.