Merge GCC frontend into the branch

gcc/README.md

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+# Elna programming language
+
+## Current implementation
+
+This repository contains a GCC frontend for Elna. After finishing the frontend
+I'm planning to rewrite the compiler in Elna itself with its own backend and
+a hand-written parser. So GCC gives a way to have a simple bootstrap compiler
+and a possbility to compile Elna programs for different platforms.
+
+## Grammar
+
+Flex and bison grammar specifications, `lexer.ll` and `parser.yy`, can be found
+in the `boot/` directory.
+
+## Build
+
+The frontend requires GCC 15.2.0 (not tested with other versions).
+
+Download the GCC source. Copy the contents of this repository into `gcc/elna`
+inside GCC. Finally build GCC enabling the frontend with
+`--enable-languages=c,c++,elna`. After the installation the compiler can be
+invoked with `$prefix/bin/gelna`.
+
+There is also a `Rakefile` that downloads, builds and installs GCC into the
+`./build/` subdirectory. The `Rakefile` assumes that ruby and rake, as well as
+all GCC dependencies are already available in the system. It works under Linux
+and Mac OS. In the latter case GCC is patched with the patches used by Homebrew
+(official GCC doesn't support Apple silicon targets). Invoke with
+
+```sh
+rake boot
+```
+
+`gcc` binary is used by default, but a different gcc version can be specified
+by passing `CC` and `CXX` environment variables to rake, e.g.:
+
+```sh
+rake CC=gcc-15 CXX=g++-15 boot
+```
+
+See `rake -T` for more tasks. The GCC source is under `build/tools`. The
+installation path is `build/host/install`.