Add /usr/local to header/library search path on BSD

libexecinfo and libintl are located in /usr/local on FreeBSD and
OpenBSD, and possibly on other BSD systems I haven't tested as well.
Makefile currently doesn't add /usr/local/include and /usr/local/lib to
header and library search paths, so users will have to manually specify
by prepending

CPLUS_INCLUDE_PATH=/usr/local/include LIBRARY_PATH=/usr/local/lib

before gmake command to compile.

Makefile

@@ -785,6 +785,10 @@ ifeq ($(BSD), 1)
   ifeq ($(LOCALIZE),1)
     LDFLAGS += -lintl -liconv
   endif
+
+  # libexecinfo, libintl and libiconv may be located in /usr/local on BSD
+  CXXFLAGS += -I/usr/local/include
+  LDFLAGS += -L/usr/local/lib
 endif
 
 # Global settings for Windows targets (at end)