meson: skip gitweb build when Perl is disabled

It is possible to configure a Git build without Perl when disabling both
our test suite and all Perl-based features. In Meson, this can be
achieved with `meson setup -Dperl=disabled -Dtests=false`.

It was reported by a user that this breaks the Meson build because
gitweb gets built even if Perl was not discovered in such a build:

    $ meson setup .. -Dtests=false -Dperl=disabled
    ...
    ../gitweb/meson.build:2:43: ERROR: Unable to get the path of a not-found external program

Fix this issue by introducing a new feature-option that allows the user
to configure whether or not to build Gitweb. The feature is set to
'auto' by default and will be disabled automatically in case Perl was
not found on the system.

Reported-by: Daniel Engberg <daniel.engberg.lists@pyret.net>
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

meson.build

@@ -740,7 +740,7 @@ endif
 # features. It is optional if you want to neither execute tests nor use any of
 # these optional features.
 perl_required = get_option('perl')
-if get_option('tests')
+if get_option('tests') or get_option('gitweb').enabled()
   perl_required = true
 endif
 
@@ -1874,7 +1874,15 @@ if intl.found()
   subdir('po')
 endif
 subdir('contrib')
-subdir('gitweb')
+
+# Gitweb requires Perl, so we disable the auto-feature if Perl was not found.
+# We make sure further up that Perl is required in case the gitweb option is
+# enabled.
+gitweb_option = get_option('gitweb').disable_auto_if(not perl.found())
+if gitweb_option.enabled()
+  subdir('gitweb')
+endif
+
 subdir('templates')
 
 # Everything but the bin-wrappers need to come before this target such that we
@@ -1893,6 +1901,7 @@ summary({
   'curl': curl.found(),
   'expat': expat.found(),
   'gettext': intl.found(),
+  'gitweb': gitweb_option.enabled(),
   'https': https_backend,
   'iconv': iconv.found(),
   'pcre2': pcre2.found(),

meson_options.txt

@@ -23,6 +23,8 @@ option('expat', type: 'feature', value: 'enabled',
   description: 'Build helpers used to push to remotes with the HTTP transport.')
 option('gettext', type: 'feature', value: 'auto',
   description: 'Build translation files.')
+option('gitweb', type: 'feature', value: 'auto',
+  description: 'Build Git web interface. Requires Perl.')
 option('iconv', type: 'feature', value: 'auto',
   description: 'Support reencoding strings with different encodings.')
 option('pcre2', type: 'feature', value: 'enabled',