possible bug in 1.4l

[Robert Collins]

The following rule

if USE_SNMP
SNMPSOURCE = snmp_core.c snmp_agent.c
endif

resulted in a makefile containing
SNMPSOURCE = snmp_core.c snmp_agent.c
when USE_SNMP was false.

the following rules

if USE_SNMP
SNMPSOURCE = snmp_core.c snmp_agent.c
else
SNMPSOURCE =
endif

worked correctly (only define SNMPSOURCE when USE_SNMP is true).

This used to work correctly, so I'd call it a regression. I'm mailing in
case this is by design, in which case I'll go hit up all my .am files :]
Otherwise, if it's a bug let me know and I'll generate a PR and
testcase.

Rob