Fix linking shared libs with ldmd2 and gdmd

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

`alias Nullable.get this` will soon be deprecated. Use `.get`.

fix #1003 - Check empty ldflags

Use -dumpfullversion to determine GDC version

Windows: Don't use Optlink by default (#1661) ...

Windows: Don't use Optlink by default
merged-on-behalf-of: Sebastian Wilzbach <sebi.wilzbach@gmail.com>

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.

Remove the additional '|' syntax for toolchain requirements. ...

- '|' seems confusing, given that it is used as a logical AND
- Dependency already supports specifying an arbitrary range and this is the only meaningful use for '|'
- Keeps the possibility open to use '|' as a logical OR in a later version
- Rewords the change log entry and adds a link to the DUB documentation

Merge pull request #1571 from rtbo/requirements ...

toolchainRequirements recipe support
merged-on-behalf-of: Nicholas Wilson <thewilsonator@users.noreply.github.com>

toolchainRequirements recipe support

extraDependencyFiles attribute added

infer unspecified version filters ...

- otherwise of little use as all dependencies need to have specified
  version filters before this can be used
- also version identifiers are fairly easy to grep for
- cache inferred version filters
- still allows explicit specification for packages that use version
  identifiers in mixins

Added experimental build settings to filter applicable version identifiers ...

Those settings allow to filter version identifiers and debug version identifiers
used during building a package. This can improve the build cache efficiency for
libraries that are used by multiple applications with different version
identifiers.

- only merge filters from source dependencies when the
  dependent has an explicit filter list as well

Pre/post run commands added

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

Add build type syntax

dub.compilers.utils: Fix platform probe code when nothing is detected ...

If determinePlatform / determineArchitecture return an empty
array (due to Dub not knowing about the target platform), the probe
file crashed during compilation due to attempting to index an array
at position 0.

Fix this by using a less unnecessarily-generic function to concatenate
elements of a JSON array.