Add Meson cross-file for WebAssembly with Emscripten

scripts/wasm.ini

+# Meson cross file for creating a WebAssembly version of the preprocessor.
+#
+# Requires emscripten to be installed.
+# Was successfully tested with emscripten 3.1.69 installed through emsdk
+# tool, as described on: https://emscripten.org/docs/getting_started/downloads.html
+# Don’t forget to source script snippet in current shell before running meson.
+#
+# Compilation creates a .wasm and .js wrapper under <builddir>/src/
+#
+# Can be run locally with node.js using:
+#  node dynare-preprocessor.js file.mod
+# NB: a version of node.js is shipped with emscripten (under the node/
+# subdirectory), but another version should also work.
+
+[binaries]
+cpp = 'em++'
+
+[host_machine]
+system = 'emscripten'
+# Could be changed to wasm64 if 4GB memory constraint is hit
+# Some background: https://v8.dev/blog/4gb-wasm-memory
+cpu_family = 'wasm32'
+cpu = 'wasm32'
+endian = 'little'
+
+[built-in options]
+# Never do a debug build, because otherwise the lack of optimisations can
+# overflow the memory capacities.
+buildtype = 'release'
+# The -fexceptions flag (for both compilation and linking) is needed for an
+# unknown reason (C++ compilers are supposed to always add exception support).
+# The -Wno-unqualified-std-cast-call flag removes many warnings about “move”
+# not being qualified with “std::” namespace.
+# The -fexperimental-library flag is needed to get std::jthread support (it was
+# supposed to no longer be necessary for LLVM 20, but for some reason we still
+# need it).
+cpp_args = [ '-fexceptions', '-Wno-unqualified-std-cast-call', '-fexperimental-library' ]
+# NODERAWFS=1 is needed for accessing the local filesystem
+cpp_link_args = [ '-s', 'NODERAWFS=1', '-fexceptions' ]
+
+[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 = '/tmp/boost_1_86_0'