Fix '-pie' build with clang

Commit 97e70a59 added the option -pie to
CFLAGS and LDFLAGS, however '-pie' is just a linker option.  That
wouldn't be a problem.  However, clang is checking for that and outputs
an error or unused argument:

error: argument unused during compilation: '-pie'
Signed-off-by: NMartin Kletzander <mkletzan@redhat.com>

m4/virt-compile-pie.m4

@@ -26,7 +26,7 @@ AC_DEFUN([LIBVIRT_COMPILE_PIE],[
          ;; dnl All code is position independent on Win32 target
       *)
       gl_COMPILER_OPTION_IF([-fPIE -DPIE -pie], [
-        PIE_CFLAGS="-fPIE -DPIE -pie"
+        PIE_CFLAGS="-fPIE -DPIE"
         PIE_LDFLAGS="-pie"
       ])
     esac