Make CMake build more compatible with FetchContent

- `HAVE_SSL_IS_QUIC` variable may be used to bypass
  `check_symbol_exists` call.

- it{,prep} targets renamed to nghttp2_it{,prep}

.github/workflows/build.yml

@@ -227,7 +227,7 @@ jobs:
       if: matrix.buildtool == 'cmake'
       run: |
         cd $NGHTTP2_CMAKE_DIR/integration-tests
-        make itprep it
+        make nghttp2_itprep nghttp2_it
 
   build-cross:
     strategy:

CMakeLists.txt

@@ -198,6 +198,7 @@ include(CheckSymbolExists)
 set(HAVE_OPENSSL    ${OPENSSL_FOUND})
 if(OPENSSL_FOUND)
   set(OPENSSL_INCLUDE_DIRS  ${OPENSSL_INCLUDE_DIR})
+  if(NOT (DEFINED HAVE_SSL_IS_QUIC))
     cmake_push_check_state()
     set(CMAKE_REQUIRED_INCLUDES   "${OPENSSL_INCLUDE_DIR}")
     set(CMAKE_REQUIRED_LIBRARIES  "${OPENSSL_LIBRARIES}")
@@ -205,10 +206,11 @@ if(OPENSSL_FOUND)
       set(CMAKE_REQUIRED_LIBRARIES "${CMAKE_REQUIRED_LIBRARIES}" "ws2_32" "bcrypt")
     endif()
     check_symbol_exists(SSL_is_quic "openssl/ssl.h" HAVE_SSL_IS_QUIC)
+    cmake_pop_check_state()
+  endif()
   if(NOT HAVE_SSL_IS_QUIC)
     message(WARNING "OpenSSL in ${OPENSSL_LIBRARIES} does not have SSL_is_quic.  HTTP/3 support cannot be enabled")
   endif()
-  cmake_pop_check_state()
 else()
   set(OPENSSL_INCLUDE_DIRS  "")
   set(OPENSSL_LIBRARIES     "")

integration-tests/CMakeLists.txt

@@ -18,7 +18,7 @@ set(EXTRA_DIST
   resp-return.rb
 )
 
-add_custom_target(itprep
+add_custom_target(nghttp2_itprep
   COMMAND go get -d -v golang.org/x/net/http2
   COMMAND go get -d -v github.com/tatsuhiro-t/go-nghttp2
   COMMAND go get -d -v golang.org/x/net/websocket
@@ -40,7 +40,7 @@ if(NOT CMAKE_CURRENT_SOURCE_DIR STREQUAL CMAKE_CURRENT_BINARY_DIR)
   endforeach()
 endif()
 
-add_custom_target(it
+add_custom_target(nghttp2_it
   COMMAND sh setenv go test -v
   DEPENDS ${GO_BUILD_FILES}
 )