Fix setenv/unsetenv link errors by implementing a local version using Windows SetEnvironmentVariable.

Untested, and does not comply with the error returns per setenv.

contrib/windows.h

@@ -45,4 +45,11 @@
 #    define strdup _strdup
 #  endif
 
+/* 
+ * setenv and unsetenv are not Windows compliant nor implemented in MinGW.
+ * These declarations get rid of the "implicit declaration warning."
+ */
+int setenv(const char *name, const char *value, int overwrite);
+void unsetenv(const char *name);
+
 #endif