gnu: cross-base: Apply gcc 8 patch to gcc >= 8.

* gnu/packages/cross-base.scm (cross-gcc): Apply gcc 8 patch to gcc >= 8.
* gnu/packages/patches/gcc-8-cross-environment-variables.patch: New file.
* gnu/local.mk (dist_patch_DATA): Add the patch.

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

gnu/local.mk

@@ -815,6 +815,7 @@ dist_patch_DATA =						\
   %D%/packages/patches/gcc-6-cross-environment-variables.patch	\
   %D%/packages/patches/gcc-6-source-date-epoch-1.patch		\
   %D%/packages/patches/gcc-6-source-date-epoch-2.patch		\
+  %D%/packages/patches/gcc-8-cross-environment-variables.patch	\
   %D%/packages/patches/gcc-8-strmov-store-file-names.patch	\
   %D%/packages/patches/gcc-9-strmov-store-file-names.patch	\
   %D%/packages/patches/gd-CVE-2018-5711.patch			\

gnu/packages/cross-base.scm

@@ -205,9 +205,10 @@ target that libc."
               (patches
                (append
                 (origin-patches (package-source xgcc))
-                (cons (if (version>=? (package-version xgcc) "6.0")
-                          (search-patch "gcc-6-cross-environment-variables.patch")
-                          (search-patch "gcc-cross-environment-variables.patch"))
+                (cons (cond
+                       ((version>=? (package-version xgcc) "8.0") (search-patch "gcc-8-cross-environment-variables.patch"))
+                       ((version>=? (package-version xgcc) "6.0") (search-patch "gcc-6-cross-environment-variables.patch"))
+                       (else  (search-patch "gcc-cross-environment-variables.patch")))
                       (cross-gcc-patches target))))
               (modules '((guix build utils)))
               (snippet

gnu/packages/patches/gcc-8-cross-environment-variables.patch

@@ -0,0 +1,67 @@
+Search path environment variables for cross-compilers.  See the discussion
+at <http://gcc.gnu.org/ml/gcc/2013-02/msg00124.html>.
+
+Note: Touch 'C_INCLUDE_PATH' et al. rather than 'CPATH', as discussed
+at <http://bugs.gnu.org/22186>.
+
+diff --git a/gcc/gcc.c b/gcc/gcc.c
+index a716f708259..dc7862f413a 100644
+--- a/gcc/gcc.c
++++ b/gcc/gcc.c
+@@ -4342,7 +4342,7 @@ process_command (unsigned int decoded_options_count,
+     }
+
+   temp = env.get (LIBRARY_PATH_ENV);
+-  if (temp && *cross_compile == '0')
++  if (temp)
+     {
+       const char *startp, *endp;
+       char *nstore = (char *) alloca (strlen (temp) + 3);
+diff --git a/gcc/incpath.c b/gcc/incpath.c
+index b11c6a57939..a66a94a04e5 100644
+--- a/gcc/incpath.c
++++ b/gcc/incpath.c
+@@ -472,8 +472,8 @@ register_include_chains (cpp_reader *pfile, const char *sysroot,
+ 			 int stdinc, int cxx_stdinc, int verbose)
+ {
+   static const char *const lang_env_vars[] =
+-    { "C_INCLUDE_PATH", "CPLUS_INCLUDE_PATH",
+-      "OBJC_INCLUDE_PATH", "OBJCPLUS_INCLUDE_PATH" };
++    { "CROSS_C_INCLUDE_PATH", "CROSS_CPLUS_INCLUDE_PATH",
++      "CROSS_OBJC_INCLUDE_PATH", "CROSS_OBJCPLUS_INCLUDE_PATH" };
+   cpp_options *cpp_opts = cpp_get_options (pfile);
+   size_t idx = (cpp_opts->objc ? 2: 0);
+
+@@ -484,7 +484,7 @@ register_include_chains (cpp_reader *pfile, const char *sysroot,
+
+   /* CPATH and language-dependent environment variables may add to the
+      include chain.  */
+-  add_env_var_paths ("CPATH", INC_BRACKET);
++  add_env_var_paths ("CROSS_CPATH", INC_BRACKET);
+   add_env_var_paths (lang_env_vars[idx], INC_SYSTEM);
+
+   target_c_incpath.extra_pre_includes (sysroot, iprefix, stdinc);
+diff --git a/gcc/system.h b/gcc/system.h
+index 4abc321c71d..d6186476024 100644
+--- a/gcc/system.h
++++ b/gcc/system.h
+@@ -1209,4 +1209,6 @@ helper_const_non_const_cast (const char *p)
+ void qsort_chk (void *, size_t, size_t, int (*)(const void *, const void *));
+ #endif
+
++#define LIBRARY_PATH_ENV "CROSS_LIBRARY_PATH"
++
+ #endif /* ! GCC_SYSTEM_H */
+diff --git a/gcc/tlink.c b/gcc/tlink.c
+index ec20bd2fa0f..cd17bdce004 100644
+--- a/gcc/tlink.c
++++ b/gcc/tlink.c
+@@ -456,7 +456,7 @@ recompile_files (void)
+   file *f;
+
+   putenv (xstrdup ("COMPILER_PATH="));
+-  putenv (xstrdup ("LIBRARY_PATH="));
++  putenv (xstrdup (LIBRARY_PATH_ENV "="));
+
+   while ((f = file_pop ()) != NULL)
+     {