change strchr() return parameter to const for new libc

src/bincimapmime/convert.h

@@ -105,7 +105,7 @@ namespace Binc {
       unsigned char c = *i;
       unsigned char d = *(i + 1);
       
-      char *t;
+      const char *t;
       if ((t = strchr(hexchars, c)) == 0)
 	return "out of range";
       n = (t - hexchars) << 4;

src/utils/base64.cpp

@@ -40,7 +40,7 @@ static const char Pad64 = '=';
 bool base64_decode(const string& in, string& out)
 {
     int io = 0, state = 0, ch = 0;
-    char *pos;
+    const char *pos;
     unsigned int ii = 0;
     out.erase();
     out.reserve(in.length());