Link the math library in the native test envs

Codec 2 is floating point throughout and pulls in expf, cosf, sqrtf, atan2f,
log10f and sincosf. glibc keeps those in libm, so the native suites failed to
link on Linux. MSYS2 folds them into libc, which is why it passed locally.

Author: caveman99

platformio.ini

@@ -16,7 +16,10 @@
 src_dir = src
 
 [env]
-build_flags = -I src
+; -lm because glibc keeps the math library separate. Codec 2 is float throughout
+; and pulls in expf, cosf, sqrtf, atan2f, log10f and sincosf; MSYS2 folds those
+; into libc, so leaving it off still links on Windows and nowhere else.
+build_flags = -I src -lm
 ; The library under test lives in src/; pio test skips src_dir without this.
 test_build_src = yes