Respect the compiler and flags supplied by the package manager.

--- a/Makefile
+++ b/Makefile
@@ -1,2 +1,6 @@
+CC ?= gcc
+CFLAGS += -std=c99 -fPIC -Wall
+LDFLAGS += -shared
+
 all:
-	gcc -I/usr/local/include  -std=c99 -shared -O2 -o stereo_widener.so stereo_widener.c -fPIC -Wall -march=native
+	$(CC) $(CFLAGS) $(LDFLAGS) -o stereo_widener.so stereo_widener.c