Before this change, when one or more of name or value are not known at
comptime, build.zig files must allocate and do the concatanation, which can be
cumbersome, and also adds a redundant allocation when name and value are
slices. The new version only does a single allocation directly in the builder's
allocator to concatonate name and value.
The origional behavior is available in defineCMacroRaw, for use in situations
such as parseing c compiler arguments.
Additionally, several places have been updated to use the new funtions.
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