configure: include -lm and -DLUA_USE_POSIX in config.mk

configure

@@ -5,7 +5,10 @@
 set -e
 
 mkf=config.mk
-cflags='-std=c99' # set here to apply to all subtrees
+# these also apply to all libraries in external/
+cflags='-std=c99'
+cppflags='-DLUA_USE_POSIX'
+ldflags='-lm'
 
 # utility functions
 
@@ -57,7 +60,7 @@ gen_CFLAGS () {
 ## flags used in the linking step
 gen_LDFLAGS () {
 	if pkg-config readline; then
-		ldflags="$(pkg-config --libs readline)"
+		ldflags="$ldflags $(pkg-config --libs readline)"
 	fi
 	if present ld.lld; then
 		ldflags="$ldflags -fuse-ld=lld"
@@ -105,11 +108,13 @@ gen_LDFLAGS
 
 printf '# generated by configure
 
-_CC      = %s
-_CFLAGS  = %s
-_LDFLAGS =%s
-'             \
-	"$cc"     \
-	"$cflags" \
-	"$ldflags"\
+_CC       = %s
+_CFLAGS   = %s
+_CPPFLAGS = %s
+_LDFLAGS  =%s
+'              \
+	"$cc"      \
+	"$cflags"  \
+	"$cppflags"\
+	"$ldflags" \
 		>"$mkf"