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Check whether CFLAGS are supported before adding

Add non-essential CFLAGS only if the compiler supports it.
Enable compiling on MSVC and Clang without error-msgs or warnings
due to non-existing parameters. Note that MSVC has not been tested
beyond the fact that it compiles.

This also makes compiling on non-x86/x86_64 architectures work in the
default configuration, by using the existance of the -msse parameter as
an indication whether SSE is supported. This may not be the best check,
but is still better than requiring manual configuration.

--- a/CMakeLists.txt

+++ b/CMakeLists.txt

@@ -1,6 +1,7 @@

 cmake_minimum_required(VERSION 3.1)

 project(libdumb C)

 include(GNUInstallDirs)

+include(CheckCCompilerFlag)

 # Bump major (== soversion) on API breakages

 set(DUMB_VERSION_MAJOR 2)

@@ -13,15 +14,42 @@

 option(BUILD_ALLEGRO4 "Build Allegro4 support" ON)

 option(USE_SSE "Use SSE instructions" ON)

-set(CMAKE_C_FLAGS "-Wall -Wno-unused-variable -Wno-unused-but-set-variable")

-add_definitions(-D_FILE_OFFSET_BITS=64)

-add_definitions(-DDUMB_DECLARE_DEPRECATED)

-set(CMAKE_C_FLAGS_DEBUG "-ggdb -DDEBUGMODE=1 -D_DEBUG")

+function(check_and_add_c_compiler_flag flag flag_variable_to_add_to)

+    string(TOUPPER "${flag}" check_name)

+    string(MAKE_C_IDENTIFIER "CC_HAS${check_name}" check_name)

+    check_c_compiler_flag("${flag}" "${check_name}")

+    if(${check_name})

+        set(${flag_variable_to_add_to} "${flag} ${${flag_variable_to_add_to}}" PARENT_SCOPE)

+    endif()

+endfunction()

+

+check_and_add_c_compiler_flag("-Wno-unused-variable" CMAKE_C_FLAGS)

+check_and_add_c_compiler_flag("-Wno-unused-but-set-variable" CMAKE_C_FLAGS)

+check_and_add_c_compiler_flag("-Wall" CMAKE_C_FLAGS)

+add_definitions("-D_FILE_OFFSET_BITS=64")

+add_definitions("-DDUMB_DECLARE_DEPRECATED")

+

+set(CMAKE_C_FLAGS_DEBUG "-DDEBUGMODE=1 -D_DEBUG")

+check_and_add_c_compiler_flag("-ggdb" CMAKE_C_FLAGS_DEBUG)

+check_and_add_c_compiler_flag("-Zi" CMAKE_C_FLAGS_DEBUG)

+

 set(CMAKE_C_FLAGS_RELEASE "-ffast-math -O2 -DNDEBUG")

-set(CMAKE_C_FLAGS_RELWITHDEBINFO "-ffast-math -g -O2 -DNDEBUG")

+

+set(CMAKE_C_FLAGS_RELWITHDEBINFO "-ffast-math -O2 -DNDEBUG")

+check_and_add_c_compiler_flag("-g" CMAKE_C_FLAGS_RELWITHDEBINFO)

+check_and_add_c_compiler_flag("-Zi" CMAKE_C_FLAGS_RELWITHDEBINFO)

+

 set(CMAKE_C_FLAGS_MINSIZEREL "-ffast-math -Os -DNDEBUG")

+

 if(USE_SSE)

-    set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -D_USE_SSE -msse")

+    check_c_compiler_flag("-msse" "CC_HAS_MSSE")

+    if(CC_HAS_MSSE)

+        message(STATUS "Compiling with SSE support")

+        set(CMAKE_C_FLAGS "-msse ${CMAKE_C_FLAGS}")

+        add_definitions("-D_USE_SSE")

+    else()

+        message(STATUS "Compiling without SSE support")

+    endif()

 endif()

 link_directories(${CMAKE_CURRENT_BINARY_DIR})