Makefile: add support for generating JSON compilation database

Tools based on LibClang [1] can make use of a 'JSON Compilation Database' [2] that keeps track of the exact options used to compile a set of source files. For example, clangd [3], which is a C language server protocol implementation, can use a JSON compilation database to determine the flags needed to compile a file so it can provide proper editor integration. As a result, editors supporting the language server protocol (such as VS Code, Emacs, or Vim, with suitable plugins) can provide better searching, integration, and refactoring tools. The Clang compiler can generate JSON fragments when compiling [4], using the `-MJ` flag. These JSON fragments (one per compiled source file) can then be concatenated to create the compilation database, commonly called 'compile_commands.json'. Add support to the Makefile for generating these JSON fragments as well as the compilation database itself, if the environment variable 'GENERATE_COMPILATION_DATABASE' is set. If this variable is set, check that $(CC) indeed supports the `-MJ` flag, following what is done for automatic dependencies. All JSON fragments are placed in the 'compile_commands/' directory, and the compilation database 'compile_commands.json' is generated as a dependency of the 'all' target using a `sed` invocation. [1] https://clang.llvm.org/docs/Tooling.html [2] https://clang.llvm.org/docs/JSONCompilationDatabase.html [3] https://clangd.llvm.org/ [4] https://clang.llvm.org/docs/ClangCommandLineReference.html#cmdoption-clang-mj-arg Helped-by: brian m. carlson <sandals@crustytoothpaste.net> Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>