Use gnuconfig package for config file replacement (#26035)

* Use gnuconfig package for config file replacement

Currently the autotools build system tries to pick up config.sub and
config.guess files from the system (in /usr/share) on arm and power.
This is introduces an implicit system dependency which we can avoid by
distributing config.guess and config.sub files in a separate package,
such as the new `gnuconfig` package which is very lightweight/text only
(unlike automake where we previously pulled these files from as a
backup). This PR adds `gnuconfig` as an unconditional build dependency
for arm and power archs.

In case the user needs a system version of config.sub and config.guess,
they are free to mark `gnuconfig` as an external package with the prefix
pointing to the directory containing the config files:

```yaml
    gnuconfig:
      externals:
      - spec: gnuconfig@master
        prefix: /tmp/tmp.ooBlkyAKdw/lol
      buildable: false
```

Apart from that, this PR gives some better instructions for users when
replacing config files goes wrong.

* Mock needs this package too now, because autotools adds a depends_on

* Add documentation

* Make patch_config_files a prop, fix the docs, add integrations tests

* Make macOS happy

var/spack/repos/builtin.mock/packages/autotools-config-replacement/package.py

@@ -0,0 +1,88 @@
+# Copyright 2013-2021 Lawrence Livermore National Security, LLC and other
+# Spack Project Developers. See the top-level COPYRIGHT file for details.
+#
+# SPDX-License-Identifier: (Apache-2.0 OR MIT)
+
+import os
+
+from spack import *
+
+
+class AutotoolsConfigReplacement(AutotoolsPackage):
+    """
+    This package features broken and working config.sub and config.guess files,
+    that should be replaced by the ones provided by gnuconfig. It allows testing
+    with / without patches and with / without substitutes available.
+    """
+
+    has_code = False
+
+    version('1.0.0')
+    variant('patch_config_files', default=False)
+    variant('gnuconfig', default=False)
+
+    depends_on('gnuconfig', type='build', when='+gnuconfig')
+
+    @property
+    def patch_config_files(self):
+        return self.spec.satisfies('+patch_config_files')
+
+    def autoreconf(self, spec, prefix):
+        pass
+
+    def configure(self, spec, prefix):
+        pass
+
+    def build(self, spec, prefix):
+        pass
+
+    def install(self, spec, prefix):
+        broken = os.path.join(self.stage.source_path, 'broken')
+        working = os.path.join(self.stage.source_path, 'working')
+        install_tree(broken, self.prefix.broken)
+        install_tree(working, self.prefix.working)
+
+    @run_before('autoreconf')
+    def create_the_package_sources(self):
+        # Creates the following file structure:
+        # ./broken/config.sub    -- not executable
+        # ./broken/config.guess  -- exectuable & exit code 1
+        # ./working/config.sub   -- executable & exit code 0
+        # ./working/config.guess -- executable & exit code 0
+        # Automatic config helper script substitution should replace the two
+        # broken scripts with those from the gnuconfig package.
+
+        broken = os.path.join(self.stage.source_path, 'broken')
+        working = os.path.join(self.stage.source_path, 'working')
+
+        mkdirp(broken)
+        mkdirp(working)
+
+        # a configure script is required
+        configure_script = join_path(self.stage.source_path, 'configure')
+        with open(configure_script, 'w') as f:
+            f.write("#!/bin/sh\nexit 0")
+        os.chmod(configure_script, 0o775)
+
+        # broken config.sub (not executable)
+        broken_config_sub = join_path(broken, 'config.sub')
+        with open(broken_config_sub, 'w') as f:
+            f.write("#!/bin/sh\nexit 0")
+
+        # broken config.guess (exectuable but with error return code)
+        broken_config_guess = join_path(broken, 'config.guess')
+        with open(broken_config_guess, 'w') as f:
+            f.write("#!/bin/sh\nexit 1")
+        os.chmod(broken_config_guess, 0o775)
+
+        # working config.sub
+        working_config_sub = join_path(working, 'config.sub')
+        with open(working_config_sub, 'w') as f:
+            f.write("#!/bin/sh\nexit 0")
+        os.chmod(working_config_sub, 0o775)
+
+        # working config.guess
+        working_config_guess = join_path(working, 'config.guess')
+        with open(working_config_guess, 'w') as f:
+            f.write("#!/bin/sh\nexit 0")
+        os.chmod(working_config_guess, 0o775)

var/spack/repos/builtin.mock/packages/gnuconfig/package.py

@@ -0,0 +1,35 @@
+# Copyright 2013-2021 Lawrence Livermore National Security, LLC and other
+# Spack Project Developers. See the top-level COPYRIGHT file for details.
+#
+# SPDX-License-Identifier: (Apache-2.0 OR MIT)
+
+import os
+
+from spack import *
+
+
+class Gnuconfig(Package):
+    """
+    The GNU config.guess and config.sub scripts versioned by timestamp.
+    This package can be used as a build dependency for autotools packages that
+    ship a tarball with outdated config.guess and config.sub files.
+    """
+
+    has_code = False
+
+    version('2021-08-14')
+
+    def install(self, spec, prefix):
+        config_sub = join_path(prefix, 'config.sub')
+        config_guess = join_path(prefix, 'config.guess')
+
+        # Create files
+        with open(config_sub, 'w') as f:
+            f.write("#!/bin/sh\necho gnuconfig version of config.sub")
+
+        with open(config_guess, 'w') as f:
+            f.write("#!/bin/sh\necho gnuconfig version of config.guess")
+
+        # Make executable
+        os.chmod(config_sub, 0o775)
+        os.chmod(config_guess, 0o775)