make(1): make ampersand in ${VAR:from=to&} an ordinary character

In SysV substitutions, wildcards are expressed with % instead of &.  The
& is not mentioned in the manual page, and having another wildcard for
the whole word would be such an obscure feature that not even pkgsrc uses
it.  The easiest way to discover this feature had been to read the source
code of make(1) or to use a fuzzer and accidentally stumble upon this
edge case.

(rillig)