Fix build when PROGRAM_DIRS is empty.

Fix build when PROGRAM_DIRS is empty.

SUBDIRS just takes PROGRAM_DIRS value.  If PROGRAM_DIRS gets set
to the empty string (as can happen when building only OSMesa), a
'for' loop will lack anything to iterate over, causing a parse
error.

This fixes the issue by making sure SUBDIRS is the null string
when PROGRAM_DIRS is, and wrapping the for loops in if's, causing
them only to execute if there are directories to iterate over.