Makefile: consistently use @PLACEHOLDER@ to substitute

Makefile: consistently use @PLACEHOLDER@ to substitute

We have a bunch of placeholders in our scripts that we replace at build
time, for example by using sed(1). These placeholders come in three
different formats: @PLACEHOLDER@, @@PLACEHOLDER@@ and ++PLACEHOLDER++.

Next to being inconsistent it also creates a bit of a problem with
CMake, which only supports the first syntax in its `configure_file()`
function. To work around that we instead manually replace placeholders
via string operations, which is a hassle and removes safeguards that
CMake has to verify that we didn't forget to replace any placeholders.
Besides that, other build systems like Meson also support the CMake
syntax.

Unify our codebase to consistently use the syntax supported by such
build systems.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>