Add new autoconf macro for adding brew directories to flags

M2/configure.ac

@@ -353,6 +353,19 @@ AC_SUBST(SIZEOF_INT_P,$ac_cv_sizeof_int_p)
 AC_CHECK_SIZEOF([long])
 AC_SUBST(SIZEOF_LONG,$ac_cv_sizeof_long)
 
+dnl Check for Homebrew packages
+
+AC_CHECK_PROGS([BREW], [brew], [false])
+
+dnl BREW_ADD_FLAG([FORMULA], [DIR], [FLAG], [PREFIX])
+dnl adds PREFIX$(brew --prefix FORMULA)/DIR (if it exists) to FLAG
+AC_DEFUN([BREW_ADD_FLAG],
+    [AS_IF([test "x$BREW" = xbrew -a -d $($BREW --prefix $1)/$2],
+         [$3="$$3 $4$($BREW --prefix $1)/$2"], [echo foo])])
+
+BREW_ADD_FLAG([], [lib],     [LDFLAGS],  [-L])
+BREW_ADD_FLAG([], [include], [CPPFLAGS], [-I])
+
 AC_CHECK_HEADERS(sys/ioctl.h termios.h sys/mman.h sys/socket.h netdb.h netinet/in.h arpa/inet.h sys/time.h time.h sys/wait.h sys/resource.h io.h linux/personality.h stddef.h elf.h execinfo.h syscall.h math.h pthread.h assert.h alloca.h malloc.h dlfcn.h)
 AC_CHECK_HEADERS(winsock2.h) dnl used with ws2_32.dll under mingw64
 dnl winsock2.h should be included before including windows.h