Allow override_find_program to use an executable.

With this it is now possible to do

foobar = executable('foobar', ...)
meson.override_find_program('foobar', foobar)

Which is convenient for a project like protobuf which produces both a
dependency and a tool. If protobuf is updated to use
override_find_program, it can be used as

protobuf_dep = dependency('protobuf', version : '>=3.3.1',
                          fallback : ['protobuf', 'protobuf_dep'])
protoc_prog = find_program('protoc')

2 files changed, 11 insertions, 1 deletions

diff --git a/docs/markdown/Reference-manual.md b/docs/markdown/Reference-manual.md
index ea11f60..9762c5d 100644
--- a/docs/markdown/Reference-manual.md
+++ b/docs/markdown/Reference-manual.md
@@ -1529,7 +1529,9 @@ the following methods.
   specifies that whenever `find_program` is used to find a program
   named `progname`, Meson should not not look it up on the system but
   instead return `program`, which may either be the result of
-  `find_program` or `configure_file`.
+  `find_program`, `configure_file` or `executable`.
+
+  If `program` is an `executable`, it cannot be used during configure.
 
 - `project_version()` returns the version string specified in
   `project` function call.
diff --git a/docs/markdown/snippets/overrideexe.md b/docs/markdown/snippets/overrideexe.md
new file mode 100644
index 0000000..59213c5
--- /dev/null
+++ b/docs/markdown/snippets/overrideexe.md
@@ -0,0 +1,8 @@
+## More flexible `override_find_program()`.
+
+It is now possible to pass an `executable` to
+`override_find_program()` if the overridden program is not used during
+configure.
+
+This is particularly useful for fallback dependencies like Protobuf
+that also provide a tool like protoc.