Fix FindExec() for case where executable is found via a relative path

Fix FindExec() for case where executable is found via a relative path
in .:/home/postgres/testversion/bin:/opt/perl5.6.1/bin:/home/postgres/bin:/usr/local/bin:/usr/bin:/opt/ansic/bin:/usr/ccs/bin:/usr/contrib/bin:/opt/nettladm/bin:/opt/pd/bin:/usr/bin/X11:/usr/contrib/bin/X11:/opt/upgrade/bin:/opt/CC/bin:/opt/langtools/bin:/opt/graphics/phigs/bin:/opt/java/bin:/bin:/opt/imake/bin:/opt/hparray/bin:/opt/aCC/bin:/opt/lrom/bin:/usr/local/nmh/bin:. (I suppose the only common case for this is '.').