Introduce better CMake OS variables (#1628)

Signed-off-by: Juan Cruz Viotti <jv@jviotti.com>

Author: jviotti

cmake/Findyaml.cmake

@@ -21,7 +21,8 @@ if(NOT Yaml_FOUND)
     target_compile_options(yaml PRIVATE -Wno-int-to-pointer-cast)
   endif()
 
-  if(LINUX)
+  if(SOURCEMETA_OS_LINUX)
+    message(STATUS "Compiling libyaml with _GNU_SOURCE")
     # See https://github.com/3DSGuy/Project_CTR/issues/122
     target_compile_definitions(yaml PRIVATE _GNU_SOURCE)
   endif()

cmake/common/variables.cmake

@@ -10,3 +10,14 @@ elseif(CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
 elseif(MSVC)
   set(SOURCEMETA_COMPILER_MSVC ON)
 endif()
+
+if(CMAKE_SYSTEM_NAME STREQUAL "Darwin")
+  set(SOURCEMETA_OS_MACOS ON)
+# It seems that in some cases, `LINUX` is not set on GNU/Linux on WSL
+elseif(LINUX OR CMAKE_SYSTEM_NAME STREQUAL "Linux")
+  set(SOURCEMETA_OS_LINUX ON)
+elseif(WIN32)
+  set(SOURCEMETA_OS_WINDOWS ON)
+elseif(${CMAKE_SYSTEM_NAME} MATCHES ".*BSD")
+  set(SOURCEMETA_OS_BSD ON)
+endif()