TryLink() a black hole (writing header/lib tests without pkgconfig)

[Benjamin Reed]

I'm trying to write a test for libgif/libungif, but the SCons Try* 
primitives are not very helpful as to what they're doing.  This is the 
test; I can't see any reason why this wouldn't work:

---(snip!)---
# Copyright Benjamin Reed 2005
# BSD license (see COPYING)
def exists(env):
    return true

def generate(env):

    from SCons.Options import Options
    import os

    optionFile = env['CACHEDIR'] + 'giflib.cache.py'
    opts = Options(optionFile)
    opts.AddOptions(
       ('CACHED_GIFLIB', 'Whether the GIF library is available'),
       ('CXXFLAGS_GIFLIB',''),
       ('LINKFLAGS_GIFLIB',''),
       )
    opts.Update(env)
    if not env['HELP'] and (env['_CONFIGURE_'] or not 
env.has_key('CACHED_GIFLIB')):
       conf = env.Configure( )
       env['CACHED_GIFLIB'] = 0

       gifsource = """
#include <gif_lib.h>

int main(int argc, char **argv) {
    if (GIF_OK != 1) {
       return 1;
    }
    return 0;
}
"""

       lastLIBS    = env['LIBS']

       for lib in [ 'gif', 'ungif' ]:

          env.Replace(LIBS = [lib])

          if conf.TryLink(gifsource, 'C'):
             print "werd"
             env['CXXFLAGS_GIFLIB'] = ['-DHAVE_GIFLIB']
             env['LINKFLAGS_GIFLIB'] = ['-l' + lib]
             env['CACHED_GIFLIB'] = 1
             env.Replace(LIBS = lastLIBS)
             break
          else:
             print "boo"
             env['CACHED_GIFLIB'] = 0
             env.Replace(LIBS = lastLIBS)

       env = conf.Finish()

       if not env['CACHED_GIFLIB']:
          print 'libgif not found.'
---(snip!)---

Both tests print "boo" even though manually it works.  I *do* rely on 
EXTRAINCLUDES and EXTRALIBS for the test to pass, are they not being set 
up for some reason?

Any ideas?