fix: support building on Alpine Linux 3.17+ (musl) without libexecinfo-dev

[M-Elsaeed]

Summary

On Alpine Linux 3.17+, libexecinfo-dev was removed because musl libc does not support execinfo.h. This causes pip install awslambdaric to fail when building from source on Alpine (no musllinux wheels are published):

backward.h:238:22: fatal error: execinfo.h: No such file or directory

Fix

Make backward.cpp compilation conditional on execinfo.h availability in the bundled aws-lambda-cpp-0.2.6 CMakeLists.txt. When execinfo.h is absent, stack trace support is disabled but all Lambda RIC functionality works correctly.

include(CheckIncludeFileCXX)
check_include_file_cxx("execinfo.h" HAVE_EXECINFO_H)
if (HAVE_EXECINFO_H)
    target_sources(${PROJECT_NAME} PRIVATE "src/backward.cpp")
else()
    message("-- execinfo.h not found, stack traces disabled (musl/Alpine Linux)")
endif()

Testing

Verified with Docker on python:3.12-alpine3.20 (autoconf 2.72, no libexecinfo-dev):

• ✅ Builds successfully
• ✅ import awslambdaric works
• ✅ No regression on glibc systems (execinfo.h found → backward.cpp compiled as before)

Context

• Fixes Error : libexecinfo-dev package not found #128
• Alpine removed libexecinfo-dev in v3.17 (Nov 2022)
• The only current workaround requires pulling packages from EOL Alpine 3.16

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.