Removal of warnings if when using make(mingw32/64)

In order to ensure that JPEGoptim is compiled warning-free (since some environments refuse to compile on warning), I have added a check if S_ISREG and S_ISDIR are already defined, since they're being defined already in mingw32s stat.h file: 

Line 165:0 : #define S_ISDIR(m) (((m) & S_IFMT) == S_IFDIR)
Which is identical to the function we have here:
Line 35:0 : #define S_ISDIR(mode)  (((mode) & S_IFMT) == S_IFDIR)

The same applies to S_ISREG, at line 169 in mingws stat.h and at line 34 in this file.

A possible different approach to get rid of the warnings would be to #undef the function and just redefine it, which might look nasty but ensures that this implementation of the function is used.

In addition to these, mingw doesn't like the "#else if" statement, which should be an "#elif" statement (Line 40:7)

win32_compat.h

@@ -31,13 +31,19 @@ extern "C" {
 #define getuid(x) 0
 #define geteuid() 0
 #define chown(outfname,st_uid,st_gid) 0
+
+#ifndef S_ISREG
 #define S_ISREG(mode)  (((mode) & S_IFMT) == S_IFREG)
+#endif
+
+#ifndef S_ISDIR
 #define S_ISDIR(mode)  (((mode) & S_IFMT) == S_IFDIR)
+#endif
 
 #ifndef HOST_TYPE
 #if _WIN64
 #define HOST_TYPE "Win64"
-#else if WIN32
+#elif WIN32
 #define HOST_TYPE "Win32"
 #endif 
 #endif