Fix capnpc extension handling on Windows

On Windows executables needs to end with .exe.
Fix this in CMakeLists.txt and update capnp detection code accordingly.

c++/src/capnp/CMakeLists.txt

@@ -182,7 +182,7 @@ if(NOT CAPNP_LITE)
   install(TARGETS capnp_tool capnpc_cpp capnpc_capnp ${INSTALL_TARGETS_DEFAULT_ARGS})
 
   # Symlink capnpc -> capnp
-  install(CODE "execute_process(COMMAND \"${CMAKE_COMMAND}\" -E create_symlink capnp \"\$ENV{DESTDIR}${CMAKE_INSTALL_FULL_BINDIR}/capnpc\")")
+  install(CODE "execute_process(COMMAND \"${CMAKE_COMMAND}\" -E create_symlink capnp${CMAKE_EXECUTABLE_SUFFIX} \"\$ENV{DESTDIR}${CMAKE_INSTALL_FULL_BINDIR}/capnpc${CMAKE_EXECUTABLE_SUFFIX}\")")
 endif()  # NOT CAPNP_LITE
 
 # Tests ========================================================================

c++/src/capnp/compiler/capnp.c++

@@ -78,7 +78,7 @@ public:
       : context(context), disk(kj::newDiskFilesystem()), loader(*this) {}
 
   kj::MainFunc getMain() {
-    if (context.getProgramName().endsWith("capnpc")) {
+    if (context.getProgramName().endsWith("capnpc") || context.getProgramName().endsWith("capnpc.exe")) {
       kj::MainBuilder builder(context, VERSION_STRING,
             "Compiles Cap'n Proto schema files and generates corresponding source code in one or "
             "more languages.");