absolute path of clang (compilers) is needed in configfile.json

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or error as following

```
./compgpt.bin --config=configfile.json tests/matmul-1/simplematrix.cc
Settings: {
"invokeai":"/Users/liao6/workspace/CompilerGPT/./scripts/gpt4/exec-openai.sh gpt-4o",
"compiler":"clang",
"compileflags":"-O3 -march=native -DNDEBUG=1",
"compilerfamily":"clang",
"optreport":"-Rpass-missed=. -c",
"leanOptReport":2,
"historyFile":"query.json",
"responseFile":"response.json",
"responseField":"choices[0].message.content",
"testScript":"",
"testRuns":1,
"testOutliers":0,
"newFileExt":"",
"inputLang":"cpp",
"outputLang":"cpp",
"systemText":"You are a compiler expert for C++ code optimization. Our goal is to improve the existing code.",
"systemTextFile":"",
"roleOfAI":"assistant",
"firstPrompt":"Given the following input code in C++:\n<>\nThe compiler optimization report is as follows:\n<>\nTask 1: Recognize the coding patterns.\nTask 2: Make pattern specific optimizations to the code. Do not use OpenMP.\nTask 3: Consider the optimization report and prioritize the missed optimizations in terms of expected improvement.\nTask 4: Use the prioritized list to improve the input code further.",
"successPrompt":"The compiler optimization report for the latest version is as follows:\n<>\nTask 1: Consider the optimization report and prioritize the missed optimizations in terms of expected improvement.\nTask 2: Use the prioritized list to improve the input code further.",
"compFailPrompt":"This version did not compile. Here are the error messages:\n<>\nTry again.",
"testFailPrompt":"This version failed the regression tests. Here are the error messages:\n<>\nTry again.",
"stopOnSuccess":false,
"iterations":1
}

CmdlineArgs: [tests/matmul-1/simplematrix.cc@α-Ω](mailto:tests/matmul-1/simplematrix.cc@%CE%B1-%CE%A9)
compile: [tests/matmul-1/simplematrix.cc@α-Ω](mailto:tests/matmul-1/simplematrix.cc@%CE%B1-%CE%A9)
compile: clang -Rpass-missed=. -c -O3 -march=native -DNDEBUG=1 tests/matmul-1/simplematrix.cc
libc++abi: terminating due to uncaught exception of type boost::process::process_error: execve failed: No such file or directory
Abort trap: 6
```

Start with configfile.json and the compiler invocation shown in the issue, then trace how the configured compiler is resolved before the compile command runs. Reproduce the execve error with tests/matmul-1/simplematrix.cc; done means a configured absolute clang path is accepted and the command no longer fails with “No such file or directory”.

Written by the indexing model from the issue text.