diff --git a/Makefile.am b/Makefile.am
index f481b8d3..1e027c6a 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -423,7 +423,7 @@ profile:
 	$(MAKE) all AM_CPPFLAGS="-pg -O2 -DNDEBUG"
 
 debug:
-	$(MAKE) all AM_CPPFLAGS="-ggdb3 -Og" CFLAGS="`patsubst -O%,,$(CFLAGS)`"
+	$(MAKE) all AM_CPPFLAGS="-ggdb3 -Og" CFLAGS="`printf ' %s ' "$(CFLAGS)"|sed -E 's#[[:space:]]-O[^[:space:]]+[[:space:]]# #g'` -ggdb3 -Og"
 
 coverage:
 	$(MAKE) all AM_CPPFLAGS="-fprofile-arcs -ftest-coverage" AM_LDFLAGS="-lgcov"