Fix compilation and linking on Freebsd

Freebsd packages and ports install header files and libraries under
/usr/local. Readline and Wxwidgets are ports.

Makefile

@@ -153,8 +153,15 @@ WXVERSION=2.8
 
 endif
 
+#
+# FreeBSD rules. Tested on 12.1.
+#
 ifeq (${OS},FreeBSD)
 
+# Readline and Wxwidgets are installed under /usr/local on Freebsd
+COMMON_CXXFLAGS=-I/usr/local/include
+COMMON_LDFLAGS=-L/usr/local/lib
+
 # This is only needed for bossash, but we can't add it to BOSSASH_LIBS here
 # because that one is redefined later.
 COMMON_SRCS+=PosixSerialPort.cpp BSDPortFactory.cpp