There are too many K&R prototypes to be practical fixing the source.

Closes: #1096833

To also fix time64-related breakage.

Closes: #1092292

Closes: #1075086

To allow setting it in the environment.

Closes: #1066605

A va_list variable can be used only once.

Closes: #992182

As is recommended by Lintian nowadays.

Or else the package will fail to build on buildds.

yy_init is undocumented and should not be set with modern versions of
flex, or else the parser will segfault because critical variables will
be NULL instead of being properly initialized.

Closes: #929239