Platforms.cmake: Minor cleanup of the BSD check

1. No need to interpolate `CMAKE_SYSTEM_NAME` in the `if`.
2. Set definitions with `add_definitions` instad of `CMAKE_CXX_FLAGS`.

CMake/Platforms.cmake

@@ -9,15 +9,15 @@ if(HAIKU)
   set(UBSAN OFF)
 endif()
 
-if(${CMAKE_SYSTEM_NAME} MATCHES "FreeBSD|OpenBSD|DragonFly|NetBSD")
-  if(${CMAKE_SYSTEM_NAME} MATCHES "NetBSD")
+if(CMAKE_SYSTEM_NAME MATCHES "FreeBSD|OpenBSD|DragonFly|NetBSD")
+  if(CMAKE_SYSTEM_NAME MATCHES "NetBSD")
     add_definitions(-D_NETBSD_SOURCE)
   else()
     add_definitions(-D_BSD_SOURCE)
     set(UBSAN OFF)
   endif()
   set(ASAN OFF)
-  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -DO_LARGEFILE=0 -Dstat64=stat -Dlstat64=lstat -Dlseek64=lseek -Doff64_t=off_t -Dfstat64=fstat -Dftruncate64=ftruncate")
+  add_definitions(-DO_LARGEFILE=0 -Dstat64=stat -Dlstat64=lstat -Dlseek64=lseek -Doff64_t=off_t -Dfstat64=fstat -Dftruncate64=ftruncate)
 endif()
 
 set(TARGET_PLATFORM host CACHE STRING "Target platform")