This change introduces an additional property that may be set on a
target to allow additional properties to be exported. Normally only a
limited number of properties are exported.
Additional properties may be exported by simply setting the
`EXPORT_PROPERTIES` property on a target that is exported.
Teach the export command to handle export sets defined by invocations
of install(TARGETS ... EXPORT foo). This makes maintenance of targets
exported to both the build tree and install tree trivial.
Commit 66b290e7 (export(): Process the export() command at generate
time., 2012-10-06 ) refactored export() so that it could evaluate
strings at generate-time. This was intended for evaluating target
properties, but that commit also removed a check for target
existence at configure-time. Restore that check and add a test for
this case.
