Revert "Remove ./configure --disable-runtime"

This reverts commit cd5a11b7a5.

.travis.yml

@@ -10,6 +10,7 @@ env:
     - RUNTIME=libicu
     - RUNTIME=libidn2
     - RUNTIME=libidn
+    - RUNTIME=no
 
 addons:
     apt:

configure.ac

@@ -96,6 +96,7 @@ AC_ARG_ENABLE(runtime,
         libicu [[default]]: IDNA2008 UTS#46 library
         libidn2:          IDNA2008 library (also needs libunistring)
         libidn:           IDNA2003 library (also needs libunistring)
+  --disable-runtime       Do not link runtime IDNA functionality
   ], [
     if test "$enableval" = "libicu" -o "$enableval" = "yes"; then
       enable_runtime=libicu
@@ -106,6 +107,8 @@ AC_ARG_ENABLE(runtime,
     elif test "$enableval" = "libidn"; then
       enable_runtime=libidn
       AC_DEFINE([WITH_LIBIDN], [1], [generate PSL data using libidn])
+    elif test "$enableval" = "no"; then
+      enable_runtime=no
     else
       AC_MSG_ERROR([Unknown value $enableval for --enable-runtime])
     fi