make(1): inline call to strchr in ValidShortVarname

It's a pity that neither GCC 5 nor GCC 10 nor Clang 9 inline this code
themselves, even though it would be easy to do.

Clang 9 at least replaces strchr with memchr, but that is still too
complicated for a simple "is this character one of these" question.

For a repeated "if (varname != ...)" instead of the switch, GCC 10
generates really boring and inefficient code, even though it is easy to
see that the order of the comparisons doesn't matter.

(rillig)