remove hardcoded -O2

Index: lib/mongoose/Makefile.am
--- lib/mongoose/Makefile.am.orig
+++ lib/mongoose/Makefile.am
@@ -1,5 +1,5 @@
 # D_ variants are defaults and should always be used
-AM_CFLAGS = @D_CFLAGS@ -std=c99 -O2
+AM_CFLAGS = @D_CFLAGS@ -std=c99
 AM_LDFLAGS = @D_LDFLAGS@
 
 noinst_LIBRARIES = libmongoose.a
