Add #define for allocation increment

[michaelrsweet]

Add a #define in mxml-private.h to control the allocation increment for mxml_set_attr and mxmlIndexNew:

#define _MXML_ALLOC_SIZE 16 // Allocation increment

[michaelrsweet]

[master 002c34f] Add MXML_ALLOC_SIZE define (Issue #318)

You can add the define to the CFLAGS environment variable prior to running the configure script to change it at build time.