std.Build.Step.ConfigHeader: improve handling of autoconf style headers

[Techatrix]

1. allow using autoconf values multiple times

While trying to port libelf to the zig build system, I noticed that their config.h.in has #undef HAVE_MEMSET twice in the same file which makes it incompatible with Zig's std.Build.Step.ConfigHeader utility. It is valid to redefine a macro if the definitions are identical.

2. skip trailing whitespace in autoconf header

While trying to port GNU Readline to the zig build system, I noticed that their config.h.in has trailing whitespace in their definition of HAVE_WCHAR_T (and other macros) which makes it incompatible with Zig's std.Build.Step.ConfigHeader utility.

3. permit inserting enum values

This change not specific to autoconf style headers but applies to std.Build.Step.ConfigHeader in general. I can split this into a separate PR if needed. I kept them together to duplicate the test coverage

One use case that would benefit from this is when inserting an enum value into a config header that is determined at runtime due to relying on the build system target.

Example from libelf

pub fn build(b: *std.Build) !void {
    const config_header = b.addConfigHeader(.{ .style = .{ .autoconf = upstream.path("config.h.in") } }, .{
        /// ...
        .__libelf_i64_t = zigIntegerToC(i64, target.result),
        .__libelf_u64_t = zigIntegerToC(u64, target.result),
        .__libelf_i32_t = zigIntegerToC(i32, target.result),
        .__libelf_u32_t = zigIntegerToC(u32, target.result),
        .__libelf_i16_t = zigIntegerToC(i16, target.result),
        .__libelf_u16_t = zigIntegerToC(u16, target.result),
        /// ...
    });
}

fn zigIntegerToC(comptime Int: type, target: std.Target) enum {
    short,
    @"unsigned short",
    int,
    @"unsigned int",
    long,
    @"unsigned long",
    @"long long",
    @"unsigned long long",
} {
    // uses std.Target.cTypeBitSize and @typeInfo(Int)
}

[andrewrk]

Thanks!