[CMake] Correctly set library prefix on Windows

Instead of adding the prefix to the output name, it should be set
separately.

Author: guitargeek

cmake/modules/RootMacros.cmake

@@ -918,14 +918,10 @@ function(ROOT_LINKER_LIBRARY library)
   if(ARG_TEST) # we are building a test, so add EXCLUDE_FROM_ALL
     set(_all EXCLUDE_FROM_ALL)
   endif()
-  set(library_name ${library})
   if(TARGET ${library})
     message(FATAL_ERROR "Target ${library} already exists.")
   endif()
   if(WIN32 AND ARG_TYPE STREQUAL SHARED AND NOT ARG_DLLEXPORT)
-    if(MSVC)
-      set(library_name ${libprefix}${library})
-    endif()
     #---create a shared library with the .def file------------------------
     add_library(${library} ${_all} SHARED ${lib_srcs})
     set_target_properties(${library} PROPERTIES WINDOWS_EXPORT_ALL_SYMBOLS TRUE)
@@ -958,7 +954,10 @@ function(ROOT_LINKER_LIBRARY library)
     endif()
   endif()
 
-  set_target_properties(${library} PROPERTIES OUTPUT_NAME ${library_name})
+  set_target_properties(${library} PROPERTIES
+      PREFIX ${libprefix}
+      IMPORT_PREFIX ${libprefix} # affects the .lib import library (MSVC)
+  )
   target_include_directories(${library} INTERFACE $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>)
   # Do not add -Dname_EXPORTS to the command-line when building files in this
   # target. Doing so is actively harmful for the modules build because it