Check for g.ppmtopng existence using "g.ppmtopng help" rather than

 just "g.ppmtopng".


git-svn-id: https://svn.osgeo.org/grass/grass/trunk@40148 15284696-431f-4ddb-bdfa-cd5b030d7da7

raster/r.colors/thumbnails.py

@@ -63,7 +63,7 @@ def rotate_ppm(srcd):
     return dstd
 
 def ppmtopng(dst, src):
-    if grass.find_program("g.ppmtopng"):
+    if grass.find_program("g.ppmtopng", ["help"]):
 	grass.run_command('g.ppmtopng', input = src, output = dst)
     elif grass.find_program("pnmtopng"):
 	fh = open(dst, 'wb')