Add explicit libm linkage to fix undefined symbol errors

[veeceey]

Summary

• Adds explicit -lm linkage to fix undefined symbol errors on certain build environments
• Resolves runtime symbol resolution failures for math functions like log10()
• Only affects non-Windows platforms

Issue

Fixes #1297

When building asyncpg on certain platforms (e.g., Amazon Linux 2023 with Clang 20), the compiled C extensions fail to load at runtime with errors like:

undefined symbol: log10 (./lib/python3.12/site-packages/asyncpg/pgproto/pgproto.cpython-312-aarch64-linux-gnu.so)

This happens because the C extensions use math functions from libm but don't explicitly link against it. On some modern toolchains (particularly Clang-based ones), libm is not implicitly linked, causing runtime symbol resolution failures even though compilation succeeds.

Running ldd on the affected .so files shows they are missing the libm.so dependency:

linux-vdso.so.1 (0x0000ffff9e066000)
libc.so.6 => /lib64/libc.so.6 (0x0000ffff9ddce000)
/lib/ld-linux-aarch64.so.1 (0x0000ffff9e019000)
undefined symbol: log10

Changes

• Modified setup.py to add -lm to LDFLAGS on non-Windows systems
• This ensures proper linkage with the math library for all C extensions

Test plan

• Builds successfully on systems where libm was previously missing
• No impact on platforms where libm was already linked implicitly
• Windows builds unaffected (libm not applicable on Windows)

The fix is minimal and follows standard C extension build practices. The -lm flag is commonly used in C extension modules that use math functions and is recognized by all major compilers (GCC, Clang, etc.).