Fix warning and add missing checks for LZO library.

m4/lzo.m4

@@ -0,0 +1,31 @@
+dnl Check to find the lzo headers/libraries
+
+AC_DEFUN(tinc_LZO,
+[
+  tinc_ac_save_CPPFLAGS="$CPPFLAGS"
+
+  AC_ARG_WITH(lzo-include,
+    [  --with-lzo-include=DIR lzo headers directory],
+    [lzo_include="$withval"
+     CFLAGS="$CFLAGS -I$withval"
+     CPPFLAGS="$CPPFLAGS -I$withval"]
+  )
+
+  AC_ARG_WITH(lzo-lib,
+    [  --with-lzo-lib=DIR     lzo library directory],
+    [lzo_lib="$withval"
+     LIBS="$LIBS -L$withval"]
+  )
+
+  AC_CHECK_HEADERS(lzo1x.h,
+    [],
+    [AC_MSG_ERROR("lzo header files not found."); break]
+  )
+
+  CPPFLAGS="$tinc_ac_save_CPPFLAGS"
+
+  AC_CHECK_LIB(lzo, lzo1x_1_compress,
+    [LIBS="$LIBS -llzo"],
+    [AC_MSG_ERROR("lzo libraries not found.")]
+  )
+])