Merge remote-tracking branch 'upstream/master' into stable

Improved ldc cross compile: ...

The folder `.dub/obj` has been moved to `.dub/build-*/obj` for an independent cross / parallel build process.
Now it is possible that two processes can build the same project with different configurations.

Add support for --arch=x86_mscoff for LDC (for DMD compatibility)

dub.compilers.ldc: Fix --rdmd with --compiler=ldc2 ...

Use two dashes to make rdmd ignore the option. ldc2 doesn't care.

Add support for '-platformlib' option. Fixes #2031 ...

This change adds the newly introduced LDC linker option '-platformlib'
to the isLinkeDFlag(...) function in ldc.d so the option to be passed to
LDC when invoked by DUB for linking.

Replace occurences of 'in ref' with 'const scope ref' ...

In support for dlang/dmd#11000

ldc: support --arch=aarch64 and powerpc64 ...

Because Meson calls e.g. `dub describe something --arch aarch64`

LDC: Fix isLinkerDFlag() detection for options specified with 2 leading hyphens

Add support for iOS/tvOS/watchOS

[dip1000] Fix compilation and test it on Travis CI

Improve cross-compilation support (replace `version` checks by BuildPlatform runtime checks) ...

Also fixes #1249. For LDC as implicit cross compiler, the file
extensions for libraries and object files etc. don't depend on the
compiler, but solely on the target. Targets selected with `--arch` are
correctly probed by dub, so that the parsed `BuildPlatform` represents
the target platform.

ldc2: Only use `-L--no-as-needed` for Linux *targets*

ldc2: Fix MSVC target detection for static libs ...

With --arch, the build (target) platform might now diverge from the host
platform. The compiler's default triple doesn't matter when
cross-compiling, so use the <name>.lib MSVC format when targeting Windows
but not MinGW.

This fixes one aspect of #1599.

Enable specifying full target triples as --arch for ldc2 ...

By treating all 'architectures' containing a `-` as `-mtriple` values.
E.g., `dub --arch=wasm32-unknown-unknown-wasm` to cross-compile to
WebAssembly. This dub switch is already used when probing the compiler,
so the build platform detection works as expected.

Also add `wasm` as detected platform and use the .wasm executable file
extension (for ldc2).

This fixes #1749 when moving the `-mtriple` option from the dub config
file to the dub command line.

Support separate linking with ldc2 ...

This is basically #1548 with an augmented set of compiler flags being
included in the linking command lines.

Restrict extra ldc2 invocation for static libs to Windows hosts ...

Don't perform the does-default-target-triple-contain-msvc check on
non-Windows hosts (which would need a hacked LLVM to return true on
other hosts).

Support a few more ldc2 command-line options

fix #1003 - Check empty ldflags

Use -dumpfullversion to determine GDC version

Perform the tool chain checks based on the compiler name. ...

Gets rid of the additional methods in the Compiler interface and moves the checks to a more central location, since other generators apart from BuildGenerator might want to perform the checks in the future. Also keeps the API "package" for now to avoid adding insufficiently though-out public APIs.

Change toolchainRequirements fields to Dependency. ...

Also removes some API clutter from Compiler and PackageRecipe.

toolchainRequirements recipe support

Merge pull request #1560 from dlang/cache_ldc_coff_status ...

Cache the COFF generation status for LDC.

Merge pull request #1542 from ohdatboi/implement_1427 ...

Add betterC build option

Merge pull request #1543 from ohdatboi/fix_1228 ...

Fix dynamicLibrary extensions on OSX. Fixes #1228

Cache the COFF generation status for LDC. ...

This can speed up the up-to-date check considerably when using LDC2 on Windows.

Fix detection for other msvc triples ...

e.g. x86-pc-win32-msvc

Add betterC build option

Fix dynamicLibrary extensions on OSX. Fixes #1228

Rename Path to NativePath. ...

Matches the changes done in vibe.d 0.8.x and vibe-core.