interpreter: add feature.disable_auto_if()

Add a method to downgrade an option to disabled if it is not used.
This is useful to avoid unnecessary search for dependencies;
for example

    dep = dependency('dep', required: get_option('feature').disable_auto_if(not foo))

can be used instead of the more verbose and complex

    if get_option('feature').auto() and not foo then
      dep = dependency('', required: false)
    else
      dep = dependency('dep', required: get_option('feature'))
    endif

or to avoid unnecessary dependency searches:

  dep1 = dependency('dep1', required: get_option('foo'))
  # dep2 is only used together with dep1
  dep2 = dependency('dep2', required: get_option('foo').disable_auto_if(not dep1.found()))
 ```

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 9 insertions, 0 deletions

diff --git a/docs/markdown/Reference-manual.md b/docs/markdown/Reference-manual.md
index 12ddfce..111a6f0 100644
--- a/docs/markdown/Reference-manual.md
+++ b/docs/markdown/Reference-manual.md
@@ -2819,6 +2819,15 @@ The following methods are defined for all [`feature` options](Build-options.md#f
 - `disabled()`: returns whether the feature was set to `'disabled'`
 - `auto()`: returns whether the feature was set to `'auto'`
 - `allowed()` *(since 0.59.0)*: returns whether the feature was set to `'enabled'` or `'auto'`
+- `disable_auto_if(value)` *(since 0.59.0)*: returns the feature, with
+  `'auto'` converted to `'disabled'` if value is true.
+
+  | Feature / Condition | True      | False |
+  | ------------------- | ----      | ----- |
+  | Enabled             | Enabled   | Enabled |
+  | Disabled            | Disabled  | Disabled |
+  | Auto                | Disabled  | Auto |
+
 - `require(value, error_message: '')` *(since 0.59.0)*: returns
   the object itself if the value is true; an error if the object is
   `'enabled'` and the value is false; a disabled feature if the object