Use --sysroot flag for clang

Although not documented, clang does support the --sysroot flag, and it
does the right thing.  Use this flag intead of -isysroot which only
applies to header file searches, not the linker.

Signed-off-by: Mans Rullgard <mans@mansr.com>
(cherry picked from commit 00ba041cb3)

configure

@@ -2051,7 +2051,7 @@ add_asflags $extra_cflags
 
 if test -n "$sysroot"; then
     case "$cc_type" in
-        gcc|llvm_gcc)
+        gcc|llvm_gcc|clang)
             add_cppflags --sysroot="$sysroot"
             add_ldflags --sysroot="$sysroot"
         ;;
@@ -2059,10 +2059,6 @@ if test -n "$sysroot"; then
             add_cppflags -I"$sysinclude"
             add_ldflags  --sysroot="$sysroot"
         ;;
-        clang)
-            add_cppflags -isysroot "$sysroot"
-            add_ldflags -isysroot "$sysroot"
-        ;;
     esac
 fi