Make sure cargo sets the soname in the cdylib

[lu-zero]

There is an open issue about it. And my minimalist solution landed.

• Have cargo provide a sane way to pass the linker line needed
• Write the linker line for Linux
• Write the linker line for MacOS
• Write the linker line for Windows
  • -gnu variant
  • -msvc variant

I'd ask @dwbuiten to help on the Window part :)

[lu-zero]

rust-lang/cargo#6298 is my attempt to solve it in a clean way.

[lu-zero]

The cargo part landed, now what remains is to come up with the makefile lines.

[lu-zero]

https://github.com/lu-zero/crav1e/tree/shared-objects contains the wip

cargo-c will hide most of this later.

[lu-zero]

On macOS:

$ otool -L simple_encoding_installed
simple_encoding_installed:
	/usr/local/lib/librav1e.0.1.0.dylib (compatibility version 0.0.0, current version 0.1.0)
	/usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1252.250.1)

[lu-zero]

On Linux:

$ ldd simple_encoding_installed
	linux-vdso64.so.1 (0x00007fff83cb0000)
	librav1e.so.0 => /usr/lib64/librav1e.so.0 (0x00007fff83aa0000)
	libc.so.6 => /lib64/libc.so.6 (0x00007fff838a0000)
	libdl.so.2 => /lib64/libdl.so.2 (0x00007fff83870000)
	libpthread.so.0 => /lib64/libpthread.so.0 (0x00007fff83820000)
	libgcc_s.so.1 => /usr/lib/gcc/powerpc64le-unknown-linux-gnu/9.1.0/libgcc_s.so.1 (0x00007fff837e0000)
	/lib64/ld64.so.2 (0x00007fff83cd0000)
	libm.so.6 => /lib64/libm.so.6 (0x00007fff836c0000)