Please support LDFLAGS

[simonjwright]

This is mainly triggered by use on macOS via Alire.

On macOS, we have to use relocatable builds because static SALs don’t work (gprbuild issue 97), and this means that we need to provide libgmp.dylib.

The macOS compilers available via Alire don’t search /usr/local by default, which means that we have to tell gprbuild where to look, and the only obvious mechanism is via LDFLAGS.

Please see also libadalang issue 958.

[simonjwright]

As in libadalang #958 I recommend re-closing this.

[pmderodat]

Understood, thank you!