build: only test libbsd linking if we have found it

configure.ac

@@ -139,22 +139,23 @@ AC_FUNC_REALLOC
 AC_FUNC_FORK
 
 # Some functions can be in libbsd
-PKG_CHECK_MODULES([libbsd], [libbsd-overlay], [], [:])
-_save_CFLAGS="$CFLAGS"
-_save_LIBS="$LIBS"
-CFLAGS="$CFLAGS $libbsd_CFLAGS"
-LIBS="$LIBS $libbsd_LIBS"
-AC_MSG_CHECKING([if libbsd can be linked correctly])
-AC_LINK_IFELSE([AC_LANG_SOURCE([[int main() { return 0; }]])],
-  [
-    AC_MSG_RESULT(yes)
-    LLDP_CFLAGS="$LLDP_CFLAGS $libbsd_CFLAGS"
-    LLDP_LDFLAGS="$LLDP_LDFLAGS $libbsd_LIBS"
-  ],[
-    AC_MSG_RESULT(no)
-    CFLAGS="$_save_CFLAGS"
-    LIBS="$_save_LIBS"
-  ])
+PKG_CHECK_MODULES([libbsd], [libbsd-overlay], [
+  _save_CFLAGS="$CFLAGS"
+  _save_LIBS="$LIBS"
+  CFLAGS="$CFLAGS $libbsd_CFLAGS"
+  LIBS="$LIBS $libbsd_LIBS"
+  AC_MSG_CHECKING([if libbsd can be linked correctly])
+  AC_LINK_IFELSE([AC_LANG_SOURCE([[int main() { return 0; }]])],
+    [
+      AC_MSG_RESULT(yes)
+      LLDP_CFLAGS="$LLDP_CFLAGS $libbsd_CFLAGS"
+      LLDP_LDFLAGS="$LLDP_LDFLAGS $libbsd_LIBS"
+    ],[
+      AC_MSG_RESULT(no)
+      CFLAGS="$_save_CFLAGS"
+      LIBS="$_save_LIBS"
+    ])
+], [:])
 
 # setproctitle may have an _init function
 AC_REPLACE_FUNCS([setproctitle])