Propagate LDFLAGS if present

[ArchangelX360]

The following PR add propagation of LDFLAGS env var in the build.rs.
We had a failure compiling jemalloc-sys crate in Bazel using an LLVM toolchain as our -fuse-ld flag was not respected by the build.rs for example.
The build.rs was trying to fall back to the OS's ld linker incorrectly, which not only did not work, but also broke hermeticity.

Closes #156

Summary by CodeRabbit

• Chores
  • Build configuration now reads and applies linker flags from the LDFLAGS environment variable (falling back to CFLAGS when unset), so build/linker options can be controlled independently and will be respected during the configure/build process.

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Welcome @ArchangelX360! It looks like this is your first PR to tikv/jemallocator 🎉

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📝 Walkthrough

Walkthrough

The jemalloc-sys build script now reads LDFLAGS from the environment into a new ldflags variable (falling back to CFLAGS if unset) and uses ldflags during the configure step so linker flags are passed separately from compiler flags.

Changes

Cohort / File(s)	Summary
jemalloc Build Configuration jemalloc-sys/build.rs	Adds ldflags env handling (set from LDFLAGS or fallback to CFLAGS) and replaces prior uses of CFLAGS with ldflags in the configure invocation to propagate linker flags.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

Poem

🐰 I hop through flags with nimble feet,
LDFLAGS found, now tidy and neat,
Linker and compiler walk their lane,
No more mixing, no more strain,
A tiny hop — the build's complete.

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: propagating LDFLAGS environment variable if present in the build configuration.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment

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[coderabbitai]

Actionable comments posted: 0

Caution

Some comments are outside the diff and can’t be posted inline due to platform limitations.

⚠️ Outside diff range comments (1)

jemalloc-sys/build.rs (1)

165-201: ⚠️ Potential issue | 🟠 Major

Track LDFLAGS changes with Cargo rerun metadata.
Reading LDFLAGS via env::var means Cargo won't rerun the build when it changes, and target-prefixed versions (e.g., X86_64_UNKNOWN_LINUX_GNU_LDFLAGS) are ignored. Use read_and_watch_env("LDFLAGS") with a fallback to cflags to correctly trigger rebuilds and honor target-specific overrides.

🔧 Suggested fix

-    let ldflags = env::var("LDFLAGS").unwrap_or_else(|_| cflags.clone());
+    let ldflags = read_and_watch_env("LDFLAGS").unwrap_or_else(|_| cflags.clone());

[BusyJay]

LGTM, thanks!