Re: Removing configure flags?

[HiPhish]

Thanks to both of you I can now at least get the configure script to run, but 
it stops because it cannot find `cat`:

starting phase `configure'
building for Linux on localhost at Tue Sep 11 10:46:19 UTC 2018
looking for /bin/sh./configure: line 89: expr: command not found
 (nope)
looking for bash./configure: line 89: expr: command not found
./configure: line 89: expr: command not found
./configure: line 89: expr: command not found
 (nope)
looking for dash./configure: line 89: expr: command not found
./configure: line 89: expr: command not found
./configure: line 89: expr: command not found
 (nope)
looking for xdg-open./configure: line 89: expr: command not found
 (nope)
looking for firefox./configure: line 89: expr: command not found
 (nope)
looking for git./configure: line 89: expr: command not found
 (nope)
looking for ronn./configure: line 89: expr: command not found
./configure: line 89: expr: command not found
 (nope)
looking for shocco./configure: line 89: expr: command not found
 (nope)
warn: ronn is not installed. cannot rebuild manpages. no biggie.
warn: shocco is not installed. cannot rebuild manpages. no biggie.
okay, looks like you have everything we need. generating config files.
writing config.mk...
./configure: line 210: cat: command not found
Backtrace:
           4 (primitive-load "/gnu/store/1ilwn4mr3nb7pi6n0w4a6nkavdf…")
In ice-9/eval.scm:
   191:35  3 (_ #f)
In srfi/srfi-1.scm:
    640:9  2 (for-each #<procedure 958500 at /gnu/store/f95ghy8mx00…> …)
In /gnu/store/f95ghy8mx00fc22nrvswvnpqlfdkf2nk-module-import/guix/build/gnu-
build-system.scm:
   799:31  1 (_ _)
In /gnu/store/f95ghy8mx00fc22nrvswvnpqlfdkf2nk-module-import/guix/build/
utils.scm:
    616:6  0 (invoke _ . _)

Here is my package definition (irrelevant parts omitted):

(package
  (name "roundup")
  (build-system gnu-build-system)
  (arguments
    `(#:phases
      (modify-phases %standard-phases
        (replace 'configure
          (lambda* (#:key inputs outputs (configure-flags '()) #:allow-other-
keys)
            (let ((out (assoc-ref outputs "out")))
              (apply invoke
                `("./configure"
                  ,(string-append "--prefix=" out)
                  ,(string-append "--sysconfdir=/etc")
                  ,@configure-flags)))))))))

From my understanding, `cat` is part of coreutils, which is already an 
implicit input of the GNU build system. So the configure script should have 
it, shouldn't it?

On a related tangent: it looks like a phase is just a function which takes 
arbitrary keyword-arguments, right? Is this documented somewhere or is it 
"read the source code"? And what exactly are `outputs` and `inputs`? It looks 
like they are association lists where the keys are names and the values are 
directories.