Fix build when an older libunistring is installed (regr. 2020-02-23).
* modules/unitypes (configure.ac): Arrange to generate the .h file if
the installed one is from a version < 0.9.11.
* modules/unicase/base (configure.ac): Likewise.
* modules/uniconv/base (configure.ac): Likewise.
* modules/unilbrk/base (configure.ac): Likewise.
* modules/uninorm/base (configure.ac): Likewise.
* modules/unistdio/base (configure.ac): Likewise.
* modules/unistr/base (configure.ac): Likewise.
* modules/uniwbrk/base (configure.ac): Likewise.
* modules/uniwidth/base (configure.ac): Likewise.
* modules/unictype/base (configure.ac): Arrange to generate the .h file
if the installed one is from a version < 0.9.8.
* modules/unigbrk/base (configure.ac): Likewise.