meson: reformat default options to workaround bug in `meson configure`

meson: reformat default options to workaround bug in `meson configure`

Since 13cb20fc46 ("meson: fix compilation with Visual Studio",
2025-01-22) it has not been possible to list build options via `meson
configure`. This is due to Meson's static analysis of build options
failing to handle constant folding, and thinking we set a totally
invalid default `-std=`.

This is reported upstream but we anyways need to work with existing
versions. It turns out there is a simple solution: turn the entire
default option into a conditional branch, which means Meson sees either
nothing, or everything.

As a result, Git users can once again see pretty-printed options before
building.

Reported-by: Ramsay Jones <ramsay@ramsayjones.plus.com>
Bug: https://github.com/mesonbuild/meson/issues/14623
Signed-off-by: Eli Schwartz <eschwartz@gentoo.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>