Explicit signed char declarations for portability.

Some platforms (ARM, PowerPC, s390x) have "unsigned char" as the default
char type, and thus the build fails for certain parts of the code where
negative values are assigned to those kind of variables.

src/osgDB/ConvertBase64.cpp

@@ -28,7 +28,7 @@ namespace osgDB
 
     int base64_decode_value(char value_in)
     {
-        static const char decoding[] = {62,-1,-1,-1,63,52,53,54,55,56,57,58,59,60,61,-1,-1,-1,-2,-1,-1,-1,0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,-1,-1,-1,-1,-1,-1,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51};
+        static const signed char decoding[] = {62,-1,-1,-1,63,52,53,54,55,56,57,58,59,60,61,-1,-1,-1,-2,-1,-1,-1,0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,-1,-1,-1,-1,-1,-1,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51};
         static const char decoding_size = sizeof(decoding);
         value_in -= 43;
         if (value_in < 0 || value_in >= decoding_size) return -1;

src/osgPlugins/osgjs/Base64

@@ -61,7 +61,7 @@ namespace base64
   extern const char* to_table;
   extern const char* to_table_end;
 
-  extern const char* from_table;
+  extern const signed char* from_table;
 
 
   template <class InputIterator, class OutputIterator>

src/osgPlugins/osgjs/Base64.cpp

@@ -18,7 +18,7 @@ namespace base64
   const char* to_table_end =
     _to_table + sizeof(_to_table);
 
-  const char _from_table[128] =
+  const signed char _from_table[128] =
   {
     -1, -1, -1, -1, -1, -1, -1, -1, // 0
     -1, -1, -1, -1, -1, -1, -1, -1, // 8
@@ -37,6 +37,6 @@ namespace base64
     41, 42, 43, 44, 45, 46, 47, 48, // 112
     49, 50, 51, -1, -1, -1, -1, -1  // 120
   };
-  const char* from_table = _from_table;
+  const signed char* from_table = _from_table;
 }