cmake: add missing definitions for SerenityOS and DOS4GW

cmake/LibraryNaming.cmake

@@ -38,6 +38,7 @@ check_symbol_exists(XASH_RISCV "build.h" XASH_RISCV)
 check_symbol_exists(XASH_RISCV_DOUBLEFP "build.h" XASH_RISCV_DOUBLEFP)
 check_symbol_exists(XASH_RISCV_SINGLEFP "build.h" XASH_RISCV_SINGLEFP)
 check_symbol_exists(XASH_RISCV_SOFTFP "build.h" XASH_RISCV_SOFTFP)
+check_symbol_exists(XASH_SERENITY "build.h" XASH_SERENITY)
 check_symbol_exists(XASH_WIN32 "build.h" XASH_WIN32)
 check_symbol_exists(XASH_WIN64 "build.h" XASH_WIN64)
 check_symbol_exists(XASH_X86 "build.h" XASH_X86)
@@ -54,10 +55,14 @@ elseif(XASH_NETBSD)
 	set(BUILDOS "netbsd")
 elseif(XASH_OPENBSD)
 	set(BUILDOS "openbsd")
-elseif(XASH_HAIKU)
-	set(BUILDOS "haiku")
 elseif(XASH_EMSCRIPTEN)
 	set(BUILDOS "emscripten")
+elseif(XASH_DOS4GW)
+	set(BUILDOS "DOS4GW")
+elseif(XASH_HAIKU)
+	set(BUILDOS "haiku")
+elseif(XASH_SERENITY)
+	set(BUILDOS "serenityos")
 else()
 	message(SEND_ERROR "Place your operating system name here! If this is a mistake, try to fix conditions above and report a bug")
 endif()