[style] Fix clang-format to version 9.

BUILDING.md

@@ -121,11 +121,11 @@ additional packages are required and are invoked by `bootstrap`:
 On Debian-based Linux distributions such as Ubuntu, these dependencies
 can be satisfied with the following:
 
-    % sudo apt-get install autoconf automake libtool
+    % sudo apt-get install autoconf automake libtool clang-format-9
 
 #### Mac OS X
 
 On Mac OS X, these dependencies can be installed and satisfied using
 [Brew](https://brew.sh/):
 
-    % brew install autoconf automake libtool
+    % brew install autoconf automake libtool llvm@9

integrations/clang/clang-format.sh

@@ -1,24 +1,24 @@
 #!/usr/bin/env bash
 
-CLANG_FORMAT_VERSION="clang-format version 6.0"
+CLANG_FORMAT_VERSION="clang-format version 9.0"
 
 die() {
     echo " *** ERROR: " $*
     exit 1
 }
 
 if which clang-format-6.0 > /dev/null; then
-    alias clang-format=clang-format-6.0
+    alias clang-format=clang-format-9.0
 elif which clang-format > /dev/null; then
     case "$(clang-format --version)" in
         "$CLANG_FORMAT_VERSION"*)
             ;;
         *)
-            die "$(clang-format --version); clang-format 6.0 required"
+            die "$(clang-format --version); clang-format 9.0 required"
             ;;
     esac
 else
-    die "clang-format 6.0 required"
+    die "clang-format 9.0 required"
 fi
 
 clang-format $@ || die