const-correctness

Add "const" to a bunch of pointers that are not written to.
mapxbase: make DBFIsValueNULL() static

It's an internal function.
simplify SfRealloc()

realloc(NULL,size) is equivalent to malloc(size).  No need for this
runtime check.
remove NULL checks before free()

free(NULL) is legal (no-op).
mapfile.c: don't redefine _GNU_SOURCE

Avoids compiler warning.