Add explicit libm linkage to fix undefined symbol errors (#1305)

When building asyncpg on certain platforms (e.g., Amazon Linux 2023
with Clang 20), the C extensions fail to load at runtime with errors
like "undefined symbol: log10". This happens because the extensions
use math functions from libm but don't explicitly link against it.

On some toolchains, libm is not implicitly linked, causing runtime
symbol resolution failures even though compilation succeeds.

This fix adds `-lm` to LDFLAGS on non-Windows systems to ensure
proper linkage with the math library.

Fixes #1297

Author: veeceey

setup.py

@@ -32,6 +32,8 @@
 
 if platform.uname().system != 'Windows':
     CFLAGS.extend(['-fsigned-char', '-Wall', '-Wsign-compare', '-Wconversion'])
+    # Link against libm (math library) for functions like log10()
+    LDFLAGS.extend(['-lm'])
 
 
 _ROOT = pathlib.Path(__file__).parent