--- a/Makefile.in
+++ b/Makefile.in
@@ -97,10 +97,14 @@ SYSCONFIG_FILE = @SYSCONFIG_FILE@
 endif
 
 # Compiler flags
+ifndef CPPFLAGS
+CPPFLAGS = @CPPFLAGS@
+endif
+CPPFLAGS += @DEFS@
 ifndef CFLAGS
 CFLAGS = @CFLAGS@
 endif
-CFLAGS += -Wall -Wstrict-prototypes -pipe -O2 -fno-tree-slp-vectorize
+CFLAGS += $(CPPFLAGS)
 
 ifndef DFLAGS
 DFLAGS = @DFLAGS@