propagate cflags and neuter hardcoded optimizations

Index: posixshell/os_BSD.sh
--- posixshell/os_BSD.sh.orig
+++ posixshell/os_BSD.sh
@@ -5,10 +5,10 @@
 os_BSD () {
 	cat <<EOF >>"${MAKE_DEF_FILE}"
 CC = cc
-CFLAGS = -O2 -Wall -pipe -std=c17
+CFLAGS = -Wall -std=c17 ${CFLAGS}
 
 CXX = c++
-CXXFLAGS = -O2 -Wall -pipe -std=c++17
+CXXFLAGS = -Wall -std=c++17 ${CXXFLAGS}
 
 # C preprocessor flags
 CPPFLAGS = -DBSD=1\\
