FindHDF5: Fix search for language-specific header file

Refactoring in commit v3.6.0-rc1~85^2 (HDF5: Refactor the use of
compiler wrappers, 2016-04-04) converted code of the form

    if(${LANGUAGE} MATCHES ...)

to

    if(LANGUAGE MATCHES ...)

However, `LANGUAGE` is a foreach() loop variable and not a normal
variable so auto-dereference does not occur.  Restore the explicit `${}`
syntax and use the new name of the loop variable that has changed since
then too.

Fixes: #16651

Modules/FindHDF5.cmake

@@ -666,9 +666,9 @@ if( NOT HDF5_FOUND )
 
     foreach(__lang IN LISTS HDF5_LANGUAGE_BINDINGS)
         # find the HDF5 include directories
-        if(LANGUAGE STREQUAL "Fortran")
+        if("${__lang}" STREQUAL "Fortran")
             set(HDF5_INCLUDE_FILENAME hdf5.mod)
-        elseif(LANGUAGE STREQUAL "CXX")
+        elseif("${__lang}" STREQUAL "CXX")
             set(HDF5_INCLUDE_FILENAME H5Cpp.h)
         else()
             set(HDF5_INCLUDE_FILENAME hdf5.h)