Prevent passing -O2 to CFLAGS.
--- a/configure.ac
+++ b/configure.ac
@@ -118,7 +118,6 @@
 AM_PROG_AR
 
 AX_FLAGS_WARN_ALL
-AX_APPEND_FLAG([-O2], [AM_CFLAGS])
 AX_APPEND_FLAG([-funroll-loops], [AM_CFLAGS])
 AX_APPEND_FLAG([-pthread], [AM_CFLAGS])
 AX_CHECK_COMPILE_FLAG([-Wunused-value], [AX_APPEND_FLAG([-Wunused-value], [AM_CFLAGS])])