bug#36882: Qemu 4.2.0 build for x86_64-linux fails

[Mathieu Othacehe <m.othacehe@gmail.com>]

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> Yup, turned out patching GCC was too difficult. I'm experimenting a
> filter over inputs passed to set-path-environment-variable in set-paths.

This is also quite tricky, because the "libc" input passed to set-paths
must not be removed from C_INCLUDE_PATH in (gnu packages commencement)
for mes packages.

So, maybe the best option is to patch QEMU directly, as proposed in
the attached patch. WDYT?

Thanks,

Mathieu

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From 6d5dea746ce7b501cdb5ba41c5eff0c9ea7eb0b0 Mon Sep 17 00:00:00 2001
From: Mathieu Othacehe <m.othacehe@gmail.com>
Date: Tue, 3 Mar 2020 12:45:33 +0100
Subject: [PATCH] gnu: qemu-minimal: Fix build.

This fixes <https://issues.guix.info/issue/36882>.

* gnu/packages/virtualization.scm (qemu-minimal)[arguments]: Add a
remove-glibc phase to remove glibc from C_INCLUDE_PATH.
---
 gnu/packages/virtualization.scm | 23 ++++++++++++++++++++++-
 1 file changed, 22 insertions(+), 1 deletion(-)

diff --git a/gnu/packages/virtualization.scm b/gnu/packages/virtualization.scm
index e16405fc9c..6ab8c3802c 100644
--- a/gnu/packages/virtualization.scm
+++ b/gnu/packages/virtualization.scm
@@ -11,6 +11,7 @@
 ;;; Copyright © 2018 Sou Bunnbu <iyzsong@member.fsf.org>
 ;;; Copyright © 2018 Julien Lepiller <julien@lepiller.eu>
 ;;; Copyright © 2019 Guy Fleury Iteriteka <hoonandon@gmail.com>
+;;; Copyright © 2020 Mathieu Othacehe <m.othacehe@gmail.com>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -124,7 +125,7 @@
                "1w38hzlw7xp05gcq1nhga7hxvndxy6dfcnzi7q2il8ff110isj6k"))))
     (build-system gnu-build-system)
     (arguments
-     '(;; Running tests in parallel can occasionally lead to failures, like:
+     `(;; Running tests in parallel can occasionally lead to failures, like:
        ;; boot_sector_test: assertion failed (signature == SIGNATURE): (0x00000000 == 0x0000dead)
        #:parallel-tests? #f
        #:configure-flags (list "--enable-usb-redir" "--enable-opengl"
@@ -135,8 +136,28 @@
                                "--audio-drv-list=alsa,pa,sdl")
        ;; Make build and test output verbose to facilitate investigation upon failure.
        #:make-flags '("V=1")
+       #:modules ((srfi srfi-1)
+                  (ice-9 match)
+                  ,@%gnu-build-system-modules)
        #:phases
        (modify-phases %standard-phases
+         ;; Work around https://issues.guix.info/issue/36882.  We need to
+         ;; remove glibc from C_INCLUDE_PATH so that the one hardcoded in GCC,
+         ;; at the bottom of GCC include search-path is used.
+         (add-after 'set-paths 'remove-glibc
+          (lambda* (#:key inputs #:allow-other-keys)
+            (let* ((filters '("libc"))
+                   (input-directories
+                    (filter-map (lambda (input)
+                                  (match input
+                                    ((name . dir)
+                                     (and (not (member name filters))
+                                          dir))))
+                                inputs)))
+              (set-path-environment-variable "C_INCLUDE_PATH"
+                                             '("include")
+                                             input-directories)
+              #t)))
          (replace 'configure
            (lambda* (#:key inputs outputs (configure-flags '())
                            #:allow-other-keys)
-- 
2.25.1