gnu: slim: Fix build failure with GCC-11

GCC-11 sniffed out a long-standing bug where a pointer was being tested for a
negative value, which is impossible. Instead, check for NULL, which is how the
error result is actually returned.

See https://github.com/iwamatsu/slim/issues/14 for details.

Fixes <https://issues.guix.gnu.org/63155>.

* gnu/packages/display-managers.scm (slim) [fix-0-pointer-comparison]: new
phase

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

gnu/packages/display-managers.scm

@@ -475,7 +475,11 @@ (define-public slim
                ;; The build system's logic here is: if "Linux", then
                 ;; "systemd".  Strip that.
                 ""))
-             #t)))
+             #t))
+         (add-before 'configure 'fix-0-pointer-comparison
+           (lambda _
+             (substitute* "panel.cpp"
+               (("WinGC < 0") "WinGC == NULL")))))
        #:configure-flags '("-DUSE_PAM=yes"
                            "-DUSE_CONSOLEKIT=no")
        #:tests? #f))