libvpx: added emcc compiler to support WASM builds

[perseoGI]

Summary

Changes to recipe: libvpx

Motivation

Described in #9865, with the purpose of cross compiling ffmpeg to Web Assembly, some recipe dependencies require some extra modifications.

Details

In the case of libvpx, the _target_name to be used on the Autotools configure script was not being set correctly.
Following a Google Discussion, and a fork of libvpx, using generic-gnu as target architecture is the way to go with this library.

To correctly compile this library, as it uses Autotools and we are not directly invoking to emconfigure, emmake, etc.
the environment variables must be set correctly:

CC=emcc
CXX=em++
AR=emar
NM=emnm
RANLIB=emranlib
STRIP=emstrip

In this case to test this recipe, I'm using the packaged emsdk recipe defined in conan-io/conan-toolchains#4.

Logs

[settings]
arch=wasm
build_type=Release
compiler=emcc
compiler.cppstd=17
compiler.libcxx=libc++
compiler.version=4.0.10
os=Emscripten
[tool_requires]
*: ninja/[*], emsdk/4.0.10
[conf]
tools.build:exelinkflags=['-sINITIAL_MEMORY=64MB']
tools.build:sharedlinkflags=['-sINITIAL_MEMORY=64MB']
tools.cmake.cmake_layout:build_folder_vars=['settings.build_type', 'settings.arch']
tools.cmake.cmaketoolchain:generator=Ninja

...

ibvpx/1.14.1 (test package): Running CMake.build()
libvpx/1.14.1 (test package): RUN: cmake --build "/Users/perseo/sources/conan-center-index/recipes/libvpx/all/test_package/build/release-wasm" -- -j14
[2/2] Linking C executable test_package.js


======== Testing the package: Executing test ========
libvpx/1.14.1 (test package): Running test()
$ node recipes/libvpx/all/test_package/build/release-wasm/test_package             
vpx version v1.14.

EDIT: This PR aims to use the same configuration (generic-gnu) than the build script used in ffmpeg.wasm
https://github.com/ffmpegwasm/ffmpeg.wasm/blob/main/build/libvpx.sh

[AbrilRBS]

The provided logs show this working correctly now, good job! :)

[perseoGI]

Adding compilation log traces with a wasm profile and a custom cmd_wrapper which redirects the output artifact to node:
libvpx-wasm.txt