`configure` sometimes saves same key multiple times in `config.mk` with differing values

[pnkfelix]

I don't mind our configure script, but it definitely has occasional warts.

For example, if I do: % ../configure --enable-local-rust --local-rust-root=/nonexistant
then I get a config.mk with the following:

CFG_ENABLE_LOCAL_RUST := 1
CFG_PREFIX           := /usr/local
CFG_LOCAL_RUST_ROOT  := /nonexistant
CFG_LLVM_ROOT        := 
CFG_JEMALLOC_ROOT    := 
[...]
CFG_LLDB_PYTHON_DIR  := /Applications/Xcode.app/Contents/SharedFrameworks/LLDB.framework/Versions/A/Resources/Python
CFG_ADB              := /Users/fklock/bin/adb
CFG_LOCAL_RUST_ROOT  := /Users/fklock
CFG_ENABLE_CLANG     := 1
CFG_USING_CLANG      := 1
....

Note that CFG_LOCAL_RUST_ROOT occurs multiple times; I believe the semantics are that the make run will pick up the last assigned value, but a reasonable human skimming the file is probably going to think that the first assignment is the only one in the file.

[pnkfelix]

(The bug here is that we do a putvar when we first extract the local-rust-root from the command line options, and then we do a second putvar when we check its validity and update it with a different value (it found one in my $PATH, in a script I keep in /Users/fklock/bin/rustc, after finding the first was invalid.)