[CMake] Link with librt only when platform toolchain has it (#34255)

Not all POSIX platforms have rt as a separate library. QNX has rt as
part of libc (same as pthreads).
Add condition to check if the library can be found and link with it only
in positive scenario.

A solution for: https://github.com/grpc/grpc/issues/34254

CMakeLists.txt

@@ -301,7 +301,10 @@ if(UNIX AND NOT HAIKU)
   find_package(Threads)
   set(_gRPC_ALLTARGETS_LIBRARIES ${CMAKE_DL_LIBS} m Threads::Threads)
   if(_gRPC_PLATFORM_LINUX OR _gRPC_PLATFORM_POSIX)
-    set(_gRPC_ALLTARGETS_LIBRARIES ${_gRPC_ALLTARGETS_LIBRARIES} rt)
+    find_library(LIBRT rt)
+    if(LIBRT)
+      set(_gRPC_ALLTARGETS_LIBRARIES ${_gRPC_ALLTARGETS_LIBRARIES} rt)
+    endif()
   endif()
 endif()
 

templates/CMakeLists.txt.template

@@ -444,7 +444,10 @@
     find_package(Threads)
     set(_gRPC_ALLTARGETS_LIBRARIES <%text>${CMAKE_DL_LIBS}</%text> m Threads::Threads)
     if(_gRPC_PLATFORM_LINUX OR _gRPC_PLATFORM_POSIX)
-      set(_gRPC_ALLTARGETS_LIBRARIES <%text>${_gRPC_ALLTARGETS_LIBRARIES}</%text> rt)
+      find_library(LIBRT rt)
+      if(LIBRT)
+        set(_gRPC_ALLTARGETS_LIBRARIES <%text>${_gRPC_ALLTARGETS_LIBRARIES}</%text> rt)
+      endif()
     endif()
   endif()