bootstrap: Add options to control use of system nghttp2

Add `bootstrap` options needed since commit 0b872fd4be (nghttp2: Build
the library within CMake for use by our curl, 2020-04-01,
v3.18.0-rc1~408^2~2).

Fixes: #20987

bootstrap

@@ -622,6 +622,8 @@ Configuration:
   --no-system-bzip2       use cmake-provided bzip2 library (default)
   --system-liblzma        use system-installed liblzma library
   --no-system-liblzma     use cmake-provided liblzma library (default)
+  --system-nghttp2        use system-installed nghttp2 library
+  --no-system-nghttp2     use cmake-provided nghttp2 library (default)
   --system-zstd           use system-installed zstd library
   --no-system-zstd        use cmake-provided zstd library (default)
   --system-libarchive     use system-installed libarchive library
@@ -866,10 +868,10 @@ while test $# != 0; do
   --init=*) cmake_init_file=`cmake_arg "$1"` ;;
   --system-libs) cmake_bootstrap_system_libs="${cmake_bootstrap_system_libs} -DCMAKE_USE_SYSTEM_LIBRARIES=1" ;;
   --no-system-libs) cmake_bootstrap_system_libs="${cmake_bootstrap_system_libs} -DCMAKE_USE_SYSTEM_LIBRARIES=0" ;;
-  --system-bzip2|--system-curl|--system-expat|--system-jsoncpp|--system-libarchive|--system-librhash|--system-zlib|--system-liblzma|--system-zstd|--system-libuv)
+  --system-bzip2|--system-curl|--system-expat|--system-jsoncpp|--system-libarchive|--system-librhash|--system-zlib|--system-liblzma|--system-nghttp2|--system-zstd|--system-libuv)
     lib=`cmake_arg "$1" "--system-"`
     cmake_bootstrap_system_libs="${cmake_bootstrap_system_libs} -DCMAKE_USE_SYSTEM_LIBRARY_`cmake_toupper $lib`=1" ;;
-  --no-system-bzip2|--no-system-curl|--no-system-expat|--no-system-jsoncpp|--no-system-libarchive|--no-system-librhash|--no-system-zlib|--no-system-liblzma|--no-system-zstd|--no-system-libuv)
+  --no-system-bzip2|--no-system-curl|--no-system-expat|--no-system-jsoncpp|--no-system-libarchive|--no-system-librhash|--no-system-zlib|--no-system-liblzma|--no-system-nghttp2|--no-system-zstd|--no-system-libuv)
     lib=`cmake_arg "$1" "--no-system-"`
     cmake_bootstrap_system_libs="${cmake_bootstrap_system_libs} -DCMAKE_USE_SYSTEM_LIBRARY_`cmake_toupper $lib`=0" ;;
   --qt-gui) cmake_bootstrap_qt_gui="1" ;;