Don't parse compiler version if toolchainRequirements is not used

[jacob-carlborg]

Compiling a project using LDC nightly build I got the follow error: Invalid SemVer format: 1.15.0git-e3e96b8 [1]. 1.15.0git-e3e96b8 is the version of LDC. The dub.json file does not contain toolchainRequirements.

[1] https://travis-ci.com/jacob-carlborg/dvm/jobs/184455053#L222

[wilzbach]

This is related to #1571 as now the compiler version is parsed on every target generation. In particular, for ldc parseDependency is called which tries to parse 1.15.0git-e3e96b8 as a SemVer dependency and thus subsequently fails.

See also:

dub/source/dub/generators/build.d

Line 57 in c707c03

checkPkgRequirements(m_project.rootPackage);

dub/source/dub/recipe/packagerecipe.d

Lines 396 to 404 in c707c03

	package bool addRequirement(ref ToolchainRequirements req, string name, string value)
	{
	switch (name) {
	default: return false;
	case "dub": req.dub = parseDependency(value); break;
	case "frontend": req.frontend = parseDMDDependency(value); break;
	case "ldc": req.ldc = parseDependency(value); break;
	case "gdc": req.gdc = parseDependency(value); break;
	case "dmd": req.dmd = parseDMDDependency(value); break;

[Geod24]

I think the issue is fixed now, as LDC has moved to a semver correct scheme, and @wilzbach has mentioned that this parsing is needed.