autoconf: fix clang detection

Now this works with and without --with-clang-srcdir, with and
without an out-of-tree build.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@149749 91177308-0d34-0410-b5e6-96231b3b80d8

autoconf/configure.ac

@@ -1570,7 +1570,7 @@ dnl Configure doxygen's configuration file
 AC_CONFIG_FILES([docs/doxygen.cfg])
 
 dnl Configure clang, if present
-if test ${clang_src_root} = ""; then
+if test "${clang_src_root}" = ""; then
   clang_src_root="$srcdir/tools/clang"
 fi
 if test -f ${clang_src_root}/README.txt; then

configure

@@ -21051,7 +21051,7 @@ ac_config_files="$ac_config_files llvm.spec"
 ac_config_files="$ac_config_files docs/doxygen.cfg"
 
 
-if test ${clang_src_root} = ""; then
+if test "${clang_src_root}" = ""; then
   clang_src_root="$srcdir/tools/clang"
 fi
 if test -f ${clang_src_root}/README.txt; then