Error out if using a not found binary in the build. Closes #469.

mesonbuild/build.py

@@ -842,6 +842,8 @@ class CustomTarget:
             if isinstance(c, str):
                 final_cmd.append(c)
             elif isinstance(c, dependencies.ExternalProgram):
+                if not c.found():
+                    raise InvalidArguments('Tried to use not found external program in a build rule.')
                 final_cmd += c.get_command()
             elif isinstance(c, BuildTarget) or isinstance(c, CustomTarget):
                 self.dependencies.append(c)

test cases/failing/28 noprog use/meson.build

@@ -0,0 +1,9 @@
+project('using not found exe', 'c')
+
+nope = find_program('nonexisting', required : false)
+
+custom_target( 'aa',
+  input: 'meson.build',
+  output: 'foobar',
+  command: [nope, '@INPUT@', '@OUTPUT@']
+)