Shim uname and pkg-config for Wasm package build

[georgestagg]

When cross-compiling packages for Wasm, ensure that the uname command outputs "Emscripten" and the pkg-config command outputs a unique list of libraries suitable for static linking.

This should improve compatibility with several R packages' configure scripts.

The output for uname has been selected based on Emscripten's own implementation of the uname syscall. Some details below.

See `emscripten-core/emscripten/system/lib/libc/emscripten_syscall_stubs.c` for implementation details. With that, the following C code:

#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/utsname.h>

int main(void) {

   struct utsname buffer;

   errno = 0;
   if (uname(&buffer) < 0) {
      perror("uname");
      exit(EXIT_FAILURE);
   }

   printf("system name = %s\n", buffer.sysname);
   printf("node name   = %s\n", buffer.nodename);
   printf("release     = %s\n", buffer.release);
   printf("version     = %s\n", buffer.version);
   printf("machine     = %s\n", buffer.machine);

   return EXIT_SUCCESS;
}

when compiled with Emscripten, outputs:

$ node main.js
system name = Emscripten
node name   = emscripten
release     = 3.1.47
version     = #1
machine     = wasm32

[georgestagg]

@jeroen How does this look?

[jeroen]

I think this looks good. You could consider setting EM_PKG_CONFIG_PATH in this shim instead of webr_env but I don't think it matters too much.

After you merge it I'll run a bunch test builds and let you know if I find any regressions.

[jeroen]

Hmm I think this use of tail is not portable. I see a lot of these:

configure: sh -c "./configure.orig --host=wasm32-unknown-emscripten"
tail: invalid option -- 'r'
Try 'tail --help' for more information.
tail: invalid option -- 'r'
Try 'tail --help' for more information.
xargs: echo: terminated by signal 13
tail: invalid option -- 'r'
Try 'tail --help' for more information.
tail: invalid option -- 'r'
Try 'tail --help' for more information.
xargs: echo: terminated by signal 13

Eg: https://github.com/r-universe-org/build-wasm/actions/runs/7198206097/job/19607201772

[georgestagg]

Ah, drat! Looks like -r is BSD and not POSIX, yet. Thanks, I'll replace it with something else.

EDIT: tail -r has been replaced with a shell function in b8415c1. Technically, tac is already available in the Linux container, but I want to be sure.