PreseedDOSCache.cmake: Add vfork check.

(djgpp actually has this symbol, believe it or not. Not sure what it could
possibly do, but it says it has it!)

cmake/PreseedDOSCache.cmake

@@ -141,6 +141,7 @@ if(CMAKE_SYSTEM_NAME STREQUAL "DOS")
       set(LIBC_HAS_TRUNC                                 "1"   CACHE INTERNAL "Have symbol trunc")
       set(LIBC_HAS_TRUNCF                                "1"   CACHE INTERNAL "Have symbol truncf")
       set(LIBC_HAS_UNSETENV                              "1"   CACHE INTERNAL "Have symbol unsetenv")
+      set(LIBC_HAS_VFORK                                 "1"   CACHE INTERNAL "Have symbol vfork")
       set(LIBC_HAS_VSNPRINTF                             "1"   CACHE INTERNAL "Have symbol vsnprintf")
       set(LIBC_HAS_VSSCANF                               "1"   CACHE INTERNAL "Have symbol vsscanf")
       set(LIBC_HAS_WCHAR_H                               "1"   CACHE INTERNAL "Have include wchar.h")