Add Meson cross-file for WebAssembly with Emscripten

scripts/wasm.ini

+# Cross-compile file for creating a WebAssembly version of the preprocessor
+# Requires emscripten to be installed
+# Creates a .wasm and .js wrapper under <builddir>/src/
+# Can be run locally with node.js using:
+#  node --no-experimental-fetch dynare-preprocessor.js
+
+[binaries]
+cpp = 'em++'
+
+[host_machine]
+system = 'emscripten'
+cpu_family = 'wasm64'
+cpu = 'wasm64'
+endian = 'little'
+
+[properties]
+# It’s necessary to use a different copy of Boost than the one under
+# /usr/include, because otherwise GCC headers confuse Clang
+#boost_root = '/path/to/boost'
+# For accessing the local filesystem
+cpp_link_args = [ '-s', 'NODERAWFS=1' ]