propagate cflags and neuter hardcoded optimizations

Index: posixshell/os_BSD.sh
--- posixshell/os_BSD.sh.orig
+++ posixshell/os_BSD.sh
@@ -9,13 +9,11 @@ SHARED_FLAGS=-DBSD=1\\
 	-DUNIX=1\\
 	-D_XOPEN_SOURCE_EXTENDED=1\\
 	-I/usr/local/include\\
-	-O2\\
-	-Wall\\
-	-pipe
+	-Wall
 CC=cc
-CFLAGS=\$(SHARED_FLAGS) -std=c17
+CFLAGS=\$(SHARED_FLAGS) -std=c17 ${CFLAGS}
 CXX=c++
-CXXFLAGS=\$(SHARED_FLAGS) -std=c++17
+CXXFLAGS=\$(SHARED_FLAGS) -std=c++17 ${CXXFLAGS}
 LDFLAGS=-L/usr/local/lib
 LDLIBS=-lcrypto\\
 	-lcurl\\