[LyX/master] Add default TransformID enum value

[Pavel Sanda]

On Wed, Nov 25, 2020 at 11:30:47PM +0200, Yuriy Skalko wrote:
> Seems like std::regex was already always used for most recent compilers, so
> how about getting rid of boost::regex


Looks reasonable from GCC point of view. Can's speak about MSVC (IIRC no one
replied about MSVC 2017, right?)

> (I've already sent a patch earlier)?

I think it missed some bits, like rmoval of LYX_USE_STD_REGEX from autotools/cmake.
Pavel