scons: Work around a scons bug when calling TryCompile.

scons: Work around a scons bug when calling TryCompile.

When calling TryCompile with an empty string for the source, useful
when just testing a build flag, some versions of SCons will apparently
create some sort of equivalence between Value('') and Value(0). That
shows itself when creating config file headers, where Value(0) is
switched with Value(''), and the header defines a macro with which
expands to nothing rather than a macro which expands to 0. Later uses
of the macro of the form:

 #if CONFIG_VARIABLE

fail because CONFIG_VARIABLE expands to nothing. If it expanded to 0
like it's supposed to, then the guarded block of code would be excluded
correctly.

Change-Id: Ie324ec5f8dd307c65745b9326a11230e10caa0bd
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/41213
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Gabe Black <gabe.black@gmail.com>