configure: use additive warnings instead of subtracting from -Wall

configure: use additive warnings instead of subtracting from -Wall

-Wall varies too much by compiler and version. rather than trying to
track all the unwanted style warnings that need to be subtracted, just
enable wanted warnings.

also, move -Wno-pointer-to-int-cast outside --enable-warnings
conditional so that it always applies, since it's turning off a
nuisance warning that's on-by-default with most compilers.