build.d: Properly translate ENABLE_LTO for other compilers than LDC

[MoonlightSentinel]

• Uses -flto when building with GDC
• Issues a warning that DMD does not support LTO instead of failing due to the unknown flag -flto=full

---

This is also a workaround because dlang/installer#492 broke the nightlies

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Testing this PR locally

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[maxhaton]

Is ldmd considered a different kind? If yes, then LGTM

[MoonlightSentinel]

Yes, HOST_DMD_KIND is set to either dmd, ldc or gdc based on the actual compiler.

See

dmd/src/build.d

Lines 1159 to 1195 in 019e5c3

	const hostDmdInfo = [env["HOST_DMD_RUN"], `-Xi=compilerInfo`, `-Xf=-`].execute();
	if (hostDmdInfo.status) // Failed, JSON output currently not supported for GDC
	{
	env["HOST_DMD_KIND"] = "gdc";
	env["HOST_DMD_VERSION"] = "v2.076";
	}
	else
	{
	/// Reads the content of a single field without parsing the entire JSON
	alias get = field => hostDmdInfo.output
	.findSplitAfter(field ~ `" : "`)[1]
	.findSplitBefore(`"`)[0];
	const ver = env["HOST_DMD_VERSION"] = get(`version`)[1 .. "vX.XXX.X".length];
	// Vendor was introduced in 2.080
	if (ver < "2.080.1")
	{
	auto name = get("binary").baseName().stripExtension();
	if (name == "ldmd2")
	name = "ldc";
	else if (name == "gdmd")
	name = "gdc";
	else
	enforce(name == "dmd", "Unknown compiler: " ~ name);
	env["HOST_DMD_KIND"] = name;
	}
	else
	{
	env["HOST_DMD_KIND"] = [
	"Digital Mars D": "dmd",
	"LDC": "ldc",
	"GNU D": "gdc"
	][get(`vendor`)];
	}

[maxhaton]

ok lets merge

[maxhaton]

Actually GDC doesn't want the full part so maybe another branch is required

[MoonlightSentinel]

Fixed, it will now use -flto for GDC