[Post-install] Add a script to check that headers are valid.

Add a script that checks all installed headers for syntax errors.
This tests if they can be included standalone.

Author: hageboeck

test/PostInstall/check-headers.sh

@@ -0,0 +1,34 @@
+#!/bin/bash
+
+# Adapted from the XRootD project with friendly permission from G. Amadio.
+#
+# This script checks that each installed ROOT header can be included individually
+# without errors. The intention is to identify which headers may have missing
+# includes, missing forward declarations, or missing header dependencies, that is,
+# headers from ROOT which it includes, but were not installed by the install target.
+
+# We need to split CXXFLAGS
+# shellcheck disable=SC2086
+
+: "${CXX:=c++}"
+: "${CXXFLAGS:=-std=c++17 -Wall -Wextra -Wno-unused-parameter -Wno-unused-const-variable}"
+: "${INCLUDE_DIR:=${1}}"
+: "${NCPU:=$(sysctl -n hw.ncpu 2>/dev/null)}"
+: "${NCPU:=$(nproc --all 2>/dev/null)}"
+
+if ! command -v "${CXX}" >/dev/null; then
+	echo "Please set CXX to a valid compiler"
+  exit 2
+fi
+if [ ! -d "${INCLUDE_DIR}" ]; then
+  echo "Usage: ${0} <ROOT include directory>"
+  echo "Alternatively, set INCLUDE_DIR in the environment"
+  exit 2
+fi
+
+
+# Check all installed headers for include errors.
+HEADERS=$(find "${INCLUDE_DIR}" -type f -name '*.h*')
+
+xargs -P ${NCPU:-1} -n 1 "${CXX}" -fsyntax-only -x c++ ${CXXFLAGS} -I"${INCLUDE_DIR}" <<< "${HEADERS}" || exit 1
+