This patch reverts upstream commit b4d3485e4fc1d:

  https://gcc.gnu.org/git/?p=gcc.git;a=commitdiff;h=b4d3485e4fc1d029e620a59deb54b3f4f3f6b209

Otherwise, GCC ends up searching the wrong target directory for cross-headers.

diff --git a/gcc/configure b/gcc/configure
--- a/gcc/configure
+++ b/gcc/configure
@@ -3689,11 +3689,6 @@ esac
 fi
 
 
-# If both --with-sysroot and --with-gxx-include-dir are passed, we interpolate
-# the former in the latter and, upon success, compute gcc_gxx_include_dir as
-# relative to the sysroot.
-gcc_gxx_include_dir_add_sysroot=0
-
 # This logic must match libstdc++-v3/acinclude.m4:GLIBCXX_EXPORT_INSTALL_INFO.
 if test x${gcc_gxx_include_dir} = x; then
   if test x${enable_version_specific_runtime_libs} = xyes; then
@@ -3705,10 +3700,15 @@ if test x${gcc_gxx_include_dir} = x; then
     fi
     gcc_gxx_include_dir="\$(libsubdir)/\$(libsubdir_to_prefix)$libstdcxx_incdir"
   fi
-elif test "${with_sysroot+set}" = set; then
+fi
+
+gcc_gxx_include_dir_add_sysroot=0
+if test "${with_sysroot+set}" = set; then
   gcc_gxx_without_sysroot=`expr "${gcc_gxx_include_dir}" : "${with_sysroot}"'\(.*\)'`
   if test "${gcc_gxx_without_sysroot}"; then
-    gcc_gxx_include_dir="${gcc_gxx_without_sysroot}"
+    if test x${with_sysroot} != x/; then
+      gcc_gxx_include_dir="${gcc_gxx_without_sysroot}"
+    fi
     gcc_gxx_include_dir_add_sysroot=1
   fi
 fi
diff --git a/gcc/configure.ac b/gcc/configure.ac
index 715fcba0482..88136e232df 100644
--- a/gcc/configure.ac
+++ b/gcc/configure.ac
@@ -204,11 +204,6 @@ no)	;;
 *)	gcc_gxx_include_dir=$with_gxx_include_dir ;;
 esac])
 
-# If both --with-sysroot and --with-gxx-include-dir are passed, we interpolate
-# the former in the latter and, upon success, compute gcc_gxx_include_dir as
-# relative to the sysroot.
-gcc_gxx_include_dir_add_sysroot=0
-
 # This logic must match libstdc++-v3/acinclude.m4:GLIBCXX_EXPORT_INSTALL_INFO.
 if test x${gcc_gxx_include_dir} = x; then
   if test x${enable_version_specific_runtime_libs} = xyes; then
@@ -220,10 +215,15 @@ if test x${gcc_gxx_include_dir} = x; then
     fi
     gcc_gxx_include_dir="\$(libsubdir)/\$(libsubdir_to_prefix)$libstdcxx_incdir"
   fi
-elif test "${with_sysroot+set}" = set; then
+fi
+
+gcc_gxx_include_dir_add_sysroot=0
+if test "${with_sysroot+set}" = set; then
   gcc_gxx_without_sysroot=`expr "${gcc_gxx_include_dir}" : "${with_sysroot}"'\(.*\)'`
   if test "${gcc_gxx_without_sysroot}"; then
-    gcc_gxx_include_dir="${gcc_gxx_without_sysroot}"
+    if test x${with_sysroot} != x/; then
+      gcc_gxx_include_dir="${gcc_gxx_without_sysroot}"
+    fi
     gcc_gxx_include_dir_add_sysroot=1
   fi
 fi
patch?id=a92c6b1a2b5b8b69f248c732db4a11ddf130f0f1'>glibc-CVE-2017-1000366-pt3.patch</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/gnu/packages/patches/glibc-CVE-2017-1000366-pt3.patch?id=a92c6b1a2b5b8b69f248c732db4a11ddf130f0f1&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2017-06-25 21:56:22 -0400'>2017-06-25</span></td><td><a href='/guix/commit/gnu/packages/patches/glibc-CVE-2017-1000366-pt3.patch?id=ffc015bea26f24d862e7e877d907fbe1ab9a9967'>gnu: glibc: Fix replacement on i686.</a><span class='msg-avail'>...<span class='msg-tooltip'>This is followup to 665d6a59161769e10b52ffcbcd5cd2db22f32681.
Fixes &lt;https://bugs.gnu.org/27489&gt;.

* gnu/packages/base.scm (glibc-2.25-patched, glibc-2.24, glibc-2.23)
(glibc-2.22): Add glibc-vectorized-strcspn-guards.patch to patches.
Move a comment where it belongs.
* gnu/packages/patches/glibc-CVE-2017-1000366-pt2.patch: Swap with ...
* gnu/packages/patches/glibc-CVE-2017-1000366-pt3.patch: ... this.
* gnu/packages/patches/glibc-vectorized-strcspn-guards.patch: New file.
* gnu/local.mk (dist_patch_DATA): Add it.  Fix formatting.
</span></span></td><td>Mark H Weaver</td></tr>
<tr><td><span title='2017-06-24 02:42:37 -0400'>2017-06-24</span></td><td><a href='/guix/commit/gnu/packages/patches/glibc-CVE-2017-1000366-pt3.patch?id=665d6a59161769e10b52ffcbcd5cd2db22f32681'>gnu: glibc: Add mitigations for CVE-2017-1000366.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/base.scm (glibc/linux)[replacement]: New field.
(glibc-2.25-patched): New variable.
(glibc-2.24, glibc-2.23, glibc-2.22, glibc-2.21)[source]: Add patches.
[replacement]: New field.
(glibc-locales)[replacement]: New field.
* gnu/packages/patches/glibc-CVE-2017-1000366-pt1.patch,
gnu/packages/patches/glibc-CVE-2017-1000366-pt2.patch,
gnu/packages/patches/glibc-CVE-2017-1000366-pt3.patch: New files.
* gnu/local.mk (dist_patch_DATA): Add them.

Modified-By: Mark H Weaver &lt;mhw@netris.org&gt;
</span></span></td><td>Efraim Flashner</td></tr>