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a method.  For example, this will add a probe system requirement:

 $build->meta->around_hook(
   probe => sub {
     my $orig = shift;
     my $build = shift;
     my $type = $orig->($build, @_);
     return $type unless $type eq 'system';
     # also require a configuration file
     if(-f '/etc/foo.conf')
     {
       return 'system';
     }
     else
     {
       return 'share';
     }
   },
 );

=head2 after_hook

 $build->meta->after_hook($hook_name, sub {
   my(@args) = @_;
   ...
 });

Execute the given code reference after the hook.  The original
arguments are passed into the code reference.

=head2 before_hook

 $build->meta->before_hook($hook_name, sub {
   my(@args) = @_;
   ...
 });

Execute the given code reference before the hook.  The original
arguments are passed into the code reference.

=head2 apply_plugin

 Alien::Build->meta->apply_plugin($name);
 Alien::Build->meta->apply_plugin($name, @args);

Apply the given plugin with the given arguments.

=head1 ENVIRONMENT

L<Alien::Build> responds to these environment variables:

=over 4

=item ALIEN_BUILD_LOG

The default log class used.  See L<Alien::Build::Log> and L<Alien::Build::Log::Default>.

=item ALIEN_BUILD_PKG_CONFIG

Override the logic in L<Alien::Build::Plugin::PkgConfig::Negotiate> which
chooses the best C<pkg-config> plugin.

=item ALIEN_BUILD_POSTLOAD

semicolon separated list of plugins to automatically load after parsing
your L<alienfile>.

=item ALIEN_BUILD_PRELOAD

semicolon separated list of plugins to automatically load before parsing
your L<alienfile>.

=item ALIEN_BUILD_RC

Perl source file which can override some global defaults for L<Alien::Build>,
by, for example, setting preload and postload plugins.

=item ALIEN_DOWNLOAD_RULE

This value determines the rules by which types of downloads are allowed.  The legal
values listed under L</download_rule>, plus C<default> which will be the default for
the current version of L<Alien::Build>.  For this version that default is C<warn>.

=item ALIEN_INSTALL_NETWORK

If set to true (the default), then network fetch will be allowed.  If set to
false, then network fetch will not be allowed.

What constitutes a local vs. network fetch is determined based on the C<start_url>
and C<local_source> meta properties.  An L<alienfile> or plugin C<could> override
this detection (possibly inappropriately), so this variable is not a substitute
for properly auditing of Perl modules for environments that require that.

=item ALIEN_INSTALL_TYPE

If set to C<share> or C<system>, it will override the system detection logic.
If set to C<default>, it will use the default setting for the L<alienfile>.
The behavior of other values is undefined.

Although the recommended way for a consumer to use an L<Alien::Base> based L<Alien>
is to declare it as a static configure and build-time dependency, some consumers
may prefer to fallback on using an L<Alien> only when the consumer itself cannot
detect the necessary package. In some cases the consumer may want the user to opt-in
to using an L<Alien> before requiring it.

To keep the interface consistent among Aliens, the consumer of the fallback opt-in
L<Alien> may fallback on the L<Alien> if the environment variable C<ALIEN_INSTALL_TYPE>
is set to any value. The rationale is that by setting this environment variable the
user is aware that L<Alien> modules may be installed and have indicated consent.
The actual implementation of this, by its nature would have to be in the consuming
CPAN module.

=item DESTDIR

This environment variable will be manipulated during a destdir install.

=item PKG_CONFIG

This environment variable can be used to override the program name for C<pkg-config>
when using the command line plugin: L<Alien::Build::Plugin::PkgConfig::CommandLine>.