gnu: netpbm: Fix a hard-coded path.

Fixes <https://bugs.gnu.org/24735>.

* gnu/packages/netpbm.scm (netpbm)[arguments]: Patch a path in the 'configure'
phase.

gnu/packages/netpbm.scm

@@ -135,7 +135,9 @@ (define-syntax drop
              ;; 'gsc', so look for that instead.
              (substitute* "converter/other/pstopnm.c"
                (("\"%s/gs\"")
-                "\"%s/gsc\"")))
+                "\"%s/gsc\"")
+               (("/usr/bin/gs")
+                (string-append (assoc-ref inputs "ghostscript") "/bin/gsc"))))
            #t))
        (add-before 'check 'setup-check
          (lambda _