Allow the user to overwrite a global -link-defaultlib-shared flag

[wilzbach]

Now that OS packers are starting to add the new -link-defaultlib-shared (see #2443) to their global ldc.conf file, this option breaks release build scripts somehow.
What do you think about:

1. Adding a CLI flag to negate this, e.g. -disable-link-defaultlib-shared? (the last seen option would take precedence)
2. Let -static overwrite the global -link-defaultlib-shared preference? At the moment one just gets an error:

Error: Can't use -link-defaultlib-shared and -static together

[kinke]

Adding a CLI flag to negate this

-link-defaultlib-shared=false is working as expected, last seen option takes precedence as usual.

Let -static overwrite the global -link-defaultlib-shared preference?

Would probably indeed be useful for distros shipping with a default -link-defaultlib-shared switch.

[wilzbach]

-link-defaultlib-shared=false is working as expected, last seen option takes precedence as usual.

Great! Just missing documentation then?

> ldc -help
...
Linking options:

  -L=<linkerflag>                         - Pass <linkerflag> to the linker
  -Xcc=<ccflag>                           - Pass <ccflag> to GCC/Clang for linking
  -defaultlib=<lib1,lib2,...>             - Default libraries to link with (overrides previous)
  -disable-linker-strip-dead              - Do not try to remove unused symbols during linking
  -link-defaultlib-debug                  - Link with debug versions of default libraries
  -link-defaultlib-shared                 - Link with shared versions of default libraries
  -linker=<lld-link|lld|gold|bfd|...>     - Linker to use
  -mscrtlib=<libcmt[d]|msvcrt[d]>         - MS C runtime library to link with
  -static                                 - Create a statically linked binary, including all system dependencies

Maybe the boolean flags could be marked with e.g. -link-defaultlib-shared=<true,false> somehow?
Not sure what the standard on this is - DMD would do this

-link-defaultlib-shared
-link-defaultlib-shared=<on,off>

(e.g.-color -> https://dlang.org/dmd#switch-color)

BTW is there any reason why ldc without args and ldc -help print different pages?

[kinke]

Maybe the boolean flags could be marked with e.g. -link-defaultlib-shared=<true,false> somehow?

It's just a default LLVM Boolean commandline option with default help formatting, which doesn't mark the Boolean options in a special way. -static is a bool too, as most other options are, so -static[=<true|TRUE|True|1|false|FALSE|False|0>] are all valid cmdline args.

BTW is there any reason why ldc without args and ldc -help print different pages?

I must confess I probably never invoked ldc2 without args and so will have to check its output. ;)

[kinke]

-static now overriding -link-defaultlib-shared & ldc2 without args printing categorized help (but still lacks the help footer, for which I didn't quickly find a solution).

[p0nce]

Hello,

What is the recommended way to say you want a static defaultlib? is it -static or -link-defaultlib-shared=false?

[kinke]

The latter, assuming you don't want to enforce static C libs too.

[p0nce]

I hadn't thought about that. Indeed what we probably need is dynamic C libs, static D runtime (on POSIX).