diff --git a/Makefile b/Makefile
index 0ec102e..caffd18 100644
--- a/Makefile
+++ b/Makefile
@@ -4,7 +4,8 @@ PREFIX = /usr/local
 MANPREFIX = ${PREFIX}/man
 
 CC       = ${_CC}
-CFLAGS   = -std=c99 ${_CFLAGS} -pedantic -Iexternal/lua -Wall -Wextra -D_DEFAULT_SOURCE
+CFLAGS   = ${_CFLAGS} -Iexternal/lua -pedantic -Wall -Wextra
+CPPFLAGS = -D_DEFAULT_SOURCE
 LDFLAGS  = -lm ${_LDFLAGS}
 
 OBJS = callisto.o lcl.o lenviron.o lextra.o lfs.o ljson.o lprocess.o util.o
diff --git a/configure b/configure
index b49e800..17f0597 100755
--- a/configure
+++ b/configure
@@ -5,6 +5,7 @@
 set -e
 
 mkf=config.mk
+cflags='-std=c99' # set here to apply to all subtrees
 
 # utility functions
 
@@ -43,9 +44,9 @@ gen_CC () {
 ## flags used in the compilation step
 gen_CFLAGS () {
 	if [ -z "$debug" ]; then
-		cflags="-O2"
+		cflags="$cflags -O3"
 	else
-		cflags="-O0 -g"
+		cflags="$cflags -O0 -g"
 	fi
 
 	for flag in $cflags; do