GlExec jobs are failing if GLEXEC_BIN is set and invalid also if GlExec is optional
If GLEXEC_BIN is set to any value different from NONE (OSG, glite, auto or a path), then the Glidein will fail if no valid glexec binary is found.
If the job does not want GlExec at all (GLIDEIN_Glexec_Use is never), then the Glidein will continue and run without GlExec.
If GlExec is not required but optional (GLIDEIN_Glexec_Use is optional or not set and GLIDEIN_REQUIRE_GLEXEC_USE if false or not set), then the Glidein could continue without using GlExec instead of failing.
Setting GLEXEC_BIN forces to run w/ GlExec unless the Job is explicitly refusing to.