unzip: fix build for cray back-end

Fixes #12007.

The Cray cc wrappers don't handle the -s flag (strip) cleanly, It's
not essential to strip the binary, so just remove the flag on Cray.

Note: the default build on Cray is for the back end and the unzip
binary won't run on the front end.  To build for FE, use something
like arch=cray-fe-x86_64.

var/spack/repos/builtin/packages/unzip/package.py

@@ -14,9 +14,12 @@ class Unzip(MakefilePackage):
 
     version('6.0', '62b490407489521db863b523a7f86375')
 
-    conflicts('platform=cray', msg='Unzip does not currently build on Cray')
+    # The Cray cc wrapper doesn't handle the '-s' flag (strip) cleanly.
+    @when('platform=cray')
+    def patch(self):
+        filter_file(r'^LFLAGS2=.*', 'LFLAGS2=', join_path('unix', 'configure'))
 
-    make_args = ['-f', 'unix/Makefile']
+    make_args = ['-f', join_path('unix', 'Makefile')]
     build_targets = make_args + ['generic']
 
     def url_for_version(self, version):