patch 9.0.2013: confusing ifdefs in if_<lang>.c

Problem:  confusing ifdefs in if_<lang>.c
Solution: refactor ifndefs to #ifdefs

if_x: Avoid using #ifndef - #else - #endif

Using #ifndef - #else - #endif is sometimes confusing.
Use #ifdef - #else - #endif instead.

closes: #13310

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: Ken Takata <kentkt@csc.jp>
diff --git a/src/if_perl.xs b/src/if_perl.xs
index 180fce6..312262a 100644
--- a/src/if_perl.xs
+++ b/src/if_perl.xs
@@ -166,7 +166,13 @@
 typedef int perl_key;
 # endif
 
-# ifndef MSWIN
+# ifdef MSWIN
+#  define PERL_PROC FARPROC
+#  define load_dll vimLoadLib
+#  define symbol_from_dll GetProcAddress
+#  define close_dll FreeLibrary
+#  define load_dll_error GetWin32Error
+# else
 #  include <dlfcn.h>
 #  define HANDLE void*
 #  define PERL_PROC void*
@@ -174,12 +180,6 @@
 #  define symbol_from_dll dlsym
 #  define close_dll dlclose
 #  define load_dll_error dlerror
-# else
-#  define PERL_PROC FARPROC
-#  define load_dll vimLoadLib
-#  define symbol_from_dll GetProcAddress
-#  define close_dll FreeLibrary
-#  define load_dll_error GetWin32Error
 # endif
 /*
  * Wrapper defines