[skip ci] Update format script to enforce clang-format version, and work on OSX

scripts/format.sh

@@ -7,8 +7,33 @@
 
 set -eu -o pipefail
 
+# Get CPU count
+OS=$(uname)
+NPROC=1
+if [[ $OS = "Linux" ]] ; then
+    NPROC=$(nproc)
+elif [[ ${OS} = "Darwin" ]] ; then
+    NPROC=$(sysctl -n hw.physicalcpu)
+fi
+
+# Discover clang-format
+if type clang-format-3.8 2> /dev/null ; then
+    CLANG_FORMAT=clang-format-3.8
+elif type clang-format 2> /dev/null ; then
+    # Clang format found, but need to check version
+    CLANG_FORMAT=clang-format
+    V=$(clang-format --version)
+    if [[ $V != *3.8* ]] ; then
+        echo "clang-format is not 3.8 (returned ${V})"
+        exit 1
+    fi
+else
+    echo "No appropriate clang-format found (expected clang-format-3.8, or clang-format)"
+    exit 1
+fi
+
 find src include unit_tests example -type f -name '*.hpp' -o -name '*.cpp' \
-  | xargs -I{} -P $(nproc) clang-format -i -style=file {}
+  | xargs -I{} -P ${NPROC} ${CLANG_FORMAT} -i -style=file {}
 
 
 dirty=$(git ls-files --modified)