Added a build script.

The build script was added to obtain the correct values of the
errno C preprocessor definitions. This way no matter what the platform
has defined them as, they will be correct in the libraries code.

build/errno.c

@@ -0,0 +1,51 @@
+#include <stdio.h>
+#include <errno.h>
+
+
+// Writes a public constant value to the desired file.
+void write_constant(FILE* file, const char* name,
+                    const char* type, const int val)
+{
+   fprintf(file, "pub const %s: %s = %d;\n", name, type, val);
+}
+
+// Writes the header of the errno file.
+void write_file_header(FILE* file)
+{
+   fprintf(file, "// This was generated by a build script to obtain\n");
+   fprintf(file, "// the correct values of C preprocessor values to\n");
+   fprintf(file, "// be used as errno constant values.\n");
+}
+
+
+int main(int argc, char *argv[])
+{
+   char* filename;
+   FILE* output_file;
+
+   // This requires a filename to be given for writing to.
+   if (argc == 2)
+   {
+      filename = argv[1];
+   }
+   else if (argc > 2 || argc < 2)
+   {
+      printf("\nThis program requires a filename be given.\n\n");
+      return 1;
+   }
+
+   // Open a file for writing.
+   output_file = fopen(filename, "w");
+
+   // Write the errno file header.
+   write_file_header(output_file);
+
+   // Write the errno constants we are trying to capture.
+   write_constant(output_file, "EDOM", "CInt", EDOM);
+   write_constant(output_file, "ERANGE", "CInt", ERANGE);
+   write_constant(output_file, "EILSEQ", "CInt", EILSEQ);
+
+   // Close the file.
+   fclose(output_file);
+   return 0;
+}