This commit adds a new configure flag `--enable-compile-commands`.  The
flag enables building a [compilation
database](https://clang.llvm.org/docs/JSONCompilationDatabase.html)
while building CRuby.  This option is intended for people who are doing
development work on CRuby using Clang, and is *not* meant for use when
doing normal builds of CRuby.

Enabling this flag will add `-MJ $@.json` to the compilation flags.  The
[`MJ`
flag](https://clang.llvm.org/docs/ClangCommandLineReference.html#cmdoption-clang-mj-arg)
takes a file argument, and writes a compilation database entry unit to
that file.  For example:

```
$ make gc.o
making gc.rbinc
generating id.h
id.h updated
/usr/bin/ruby --disable=gems ./tool/gen_dummy_probes.rb ./probes.d > probes.dmyh
translating probes probes.d
. ./vm_opts.h
compiling gc.c
$ cat gc.o.json
{ "directory": "/Users/aaron/git/ruby", "file": "gc.c", "output": "gc.o", "arguments": ["/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang", "-xc", "gc.c", "-fdeclspec", "-O3", "-fno-fast-math", "-ggdb3", "-Wall", "-Wextra", "-Wextra-tokens", "-Wdeprecated-declarations", "-Wdivision-by-zero", "-Wdiv-by-zero", "-Wimplicit-function-declaration", "-Wimplicit-int", "-Wmisleading-indentation", "-Wpointer-arith", "-Wshorten-64-to-32", "-Wwrite-strings", "-Wold-style-definition", "-Wmissing-noreturn", "-Wno-cast-function-type", "-Wno-constant-logical-operand", "-Wno-long-long", "-Wno-missing-field-initializers", "-Wno-overlength-strings", "-Wno-parentheses-equality", "-Wno-self-assign", "-Wno-tautological-compare", "-Wno-unused-parameter", "-Wno-unused-value", "-Wunused-variable", "-Wundef", "-std=gnu99", "-pipe", "-D", "_FORTIFY_SOURCE=2", "-fstack-protector-strong", "-fno-strict-overflow", "-fvisibility=hidden", "-D", "RUBY_EXPORT", "-fPIE", "-I", ".", "-I", ".ext/include/arm64-darwin21", "-I", "./include", "-I", ".", "-I", "./enc/unicode/14.0.0", "-D", "_XOPEN_SOURCE", "-D", "_DARWIN_C_SOURCE", "-D", "_DARWIN_UNLIMITED_SELECT", "-D", "_REENTRANT", "-o", "gc.o", "-c", "-Wno-reorder-init-list", "-Wno-implicit-int-float-conversion", "-Wno-c99-designator", "-Wno-final-dtor-non-final-class", "-Wno-extra-semi-stmt", "-Wno-quoted-include-in-framework-header", "-Wno-implicit-fallthrough", "-Wno-enum-enum-conversion", "-Wno-enum-float-conversion", "-Wno-elaborated-enum-base", "-Wno-reserved-identifier", "-Wno-gnu-folding-constant", "-Wno-objc-load-method", "-Xclang", "-clang-vendor-feature=+messageToSelfInClassMethodIdReturnType", "-Xclang", "-clang-vendor-feature=+disableInferNewAvailabilityFromInit", "-Xclang", "-clang-vendor-feature=+disableNonDependentMemberExprInCurrentInstantiation", "-mllvm", "-disable-aligned-alloc-awareness=1", "-Xclang", "-fno-odr-hash-protocols", "-Xclang", "-clang-vendor-feature=+enableAggressiveVLAFolding", "-Xclang", "-clang-vendor-feature=+revert09abecef7bbf", "-Xclang", "-clang-vendor-feature=+thisNoAlignAttr", "-Xclang", "-clang-vendor-feature=+thisNoNullAttr", "-mlinker-version=763", "-isysroot", "/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk", "-mmacosx-version-min=12.0.0", "-stdlib=libc++", "--target=arm64-apple-macosx12.0.0"]},
```

`clangd` [looks for a file called
`compile_commands.json`](https://clangd.llvm.org/installation.html#compile_commandsjson),
and doing `make compile_commands.json` will generate that file.