find_program: Only store successful lookups

Otherwise we can't do the following workflow:

if not find_program('foo', required : false).found()
  subproject('provides-foo')
endif

Where 'provides-foo' has a meson.override_find_program() on
a configure_file() or similar.

mesonbuild/interpreter.py

@@ -2324,7 +2324,6 @@ to directly access options of other subprojects.''')
         required = kwargs.get('required', True)
         if not isinstance(required, bool):
             raise InvalidArguments('"required" argument must be a boolean.')
-        self.store_name_lookups(args)
         progobj = self.program_from_overrides(args)
         if progobj is None and self.build.environment.is_cross_build():
             use_native = kwargs.get('native', False)
@@ -2338,6 +2337,8 @@ to directly access options of other subprojects.''')
             raise InvalidArguments('Program(s) {!r} not found or not executable'.format(args))
         if progobj is None:
             return ExternalProgramHolder(dependencies.NonExistingExternalProgram())
+        # Only store successful lookups
+        self.store_name_lookups(args)
         return progobj
 
     def func_find_library(self, node, args, kwargs):

test cases/common/182 find override/meson.build

@@ -1,4 +1,12 @@
 project('find program override', 'c')
 
+gencodegen = find_program('gencodegen', required : false)
+
+assert(not gencodegen.found(), 'gencodegen is an internal program, should not be found')
+
+# Test the check-if-found-else-override workflow
+if not gencodegen.found()
   subdir('subdir')
+endif
+
 subdir('otherdir')