test/synth: define _XOPEN_SOURCE
test/synth: define _XOPEN_SOURCE

On some systems this is necessary to get the prototype for putenv().

Signed-off-by: Benjamin Gilbert <55d1d0083477f6dd5757d8929dd3b9e09d80bd84@cs.cmu.edu>

openslide/openslide,openslide/openslide,openslide/openslide,openslide/openslide