GoogleTest build fixes for Windows

In ext/googletest.cmd, use Ninja as the CMake generator because the
default CMake generator for Windows is Visual Studio (as opposed to Unix
Makefiles).

In tests/CMakeLists.txt, use ${CMAKE_STATIC_LIBRARY_PREFIX} instead of
"lib" in gtest static library names).
diff --git a/ext/googletest.cmd b/ext/googletest.cmd
index 2e2587b..f080a0f 100755
--- a/ext/googletest.cmd
+++ b/ext/googletest.cmd
@@ -11,5 +11,5 @@
 cd googletest
 mkdir build
 cd build
-cmake .. -DCMAKE_BUILD_TYPE=Release -DBUILD_GMOCK=OFF
+cmake -G Ninja -DCMAKE_BUILD_TYPE=Release -DBUILD_GMOCK=OFF ..
 cmake --build .
diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index 869db35..2c8bbdb 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -52,8 +52,8 @@
 if(AVIF_ENABLE_GTEST)
     if(AVIF_LOCAL_GTEST)
         set(GTEST_INCLUDE_DIRS ${CMAKE_SOURCE_DIR}/ext/googletest/googletest/include)
-        set(GTEST_LIBRARIES ${CMAKE_SOURCE_DIR}/ext/googletest/build/lib/libgtest${CMAKE_STATIC_LIBRARY_SUFFIX})
-        set(GTEST_MAIN_LIBRARIES ${CMAKE_SOURCE_DIR}/ext/googletest/build/lib/libgtest_main${CMAKE_STATIC_LIBRARY_SUFFIX})
+        set(GTEST_LIBRARIES ${CMAKE_SOURCE_DIR}/ext/googletest/build/lib/${CMAKE_STATIC_LIBRARY_PREFIX}gtest${CMAKE_STATIC_LIBRARY_SUFFIX})
+        set(GTEST_MAIN_LIBRARIES ${CMAKE_SOURCE_DIR}/ext/googletest/build/lib/${CMAKE_STATIC_LIBRARY_PREFIX}gtest_main${CMAKE_STATIC_LIBRARY_SUFFIX})
         set(GTEST_BOTH_LIBRARIES ${GTEST_LIBRARIES} ${GTEST_MAIN_LIBRARIES})
         if(NOT EXISTS ${GTEST_INCLUDE_DIRS}/gtest/gtest.h)
             message(FATAL_ERROR "googletest(AVIF_LOCAL_GTEST): ${GTEST_INCLUDE_DIRS}/gtest/gtest.h is missing, bailing out")