Use a linker script for libstdc++.

[Alexhuszagh]

Use a linker script for libstdc++.so since we remove the dylib, to ensure a static build is always used to avoid segfaults at runtime. However, the archive can reference missing symbols present in libc or libgcc, and itself depends on symbols in libm. To ensure these libraries are properly linked when forcing a build against the static libstdc++, create a linker script for libstdc++.so that links to the static archive, and as needed, all dependencies.

Related to #1124.

[Alexhuszagh]

So this is a much better solution than what I was doing before: to fix #902, we removed the shared libstdc++.so, however, this has issues since the static version reference symbols resolved in libc, libgcc, and itself depends on libm. To avoid any issues with detecting missing symbols in compiler-builtins, and to avoid using libgcc whenever it is not required, we only want to link to libgcc when needed, and linking another time to libc shouldn't be required. Finally, manually detecting the compiler-builtins path and linking to it uses that library for something it was never intended for: using it as libgcc so we don't have to link to libgcc.

The solution is actually quite simple: use a linker script (which I should have though of, a long time ago):

/* cross-rs linker script
 * this allows us to statically link libstdc++ to avoid segfaults
 * https://github.com/cross-rs/cross/issues/902
 */
GROUP ( libstdc++.a AS_NEEDED( -lgcc -lc -lm ) )

This way, we always link to our static C++ library, and then resolve the missing symbols if required. This works when built against a crate using re2 as a CMake dependency as seen in #1112.

[Alexhuszagh]

bors try --target musl

[bors]

try

Build succeeded:

• conclusion

[Alexhuszagh]

Just a ping for a review on this: does this look ready?

[Emilgardis]

oops my bad!

yes this looks great

bors r+

[bors]

Build succeeded:

• conclusion