Clang: All versions know about -fPIE (#13550)

The GNU compiler information file tests for GNU >= 3.4 because earlier
versions do not have the flag.  The version number test is not valid for
Clang compiler versions, but we know Clang supports the flag.

Modules/Compiler/Clang.cmake

@@ -22,4 +22,5 @@ include(Compiler/GNU)
 
 macro(__compiler_clang lang)
   __compiler_gnu(${lang})
+  set(CMAKE_${lang}_COMPILE_OPTIONS_PIE "-fPIE")
 endmacro()