[pseudo,pseudo-1.9] PATCH 04/10] configure: Require GCC 5 or later

diff --git a/README b/README
index c4cb997..039f960 100644
--- a/README
+++ b/README
@@ -11,6 +11,9 @@  environment, allowing ordinary users to run commands which give the illusion
 of creating device nodes, changing file ownership, and otherwise doing
 things necessary for creating distribution packages or filesystems.
 
+Building pseudo requires GCC 5 or later (or clang), which the configure
+script checks for.
+
 To configure, run the provided configure script.  Note that this is
 NOT an autoconf script.
 
diff --git a/configure b/configure
index 68c9962..f561dcb 100755
--- a/configure
+++ b/configure
@@ -232,6 +232,31 @@  if [ -z "$opt_sqlite_lib" ]; then
     opt_sqlite_lib=$opt_lib
 fi
 
+# pseudo uses __has_include, which needs GCC 5 or later.  clang reports
+# itself as GCC 4.2.1, so only check its support for __has_include.
+if ! cc_version=$(${CC:-cc} -dumpversion 2>/dev/null); then
+    echo >&2 "Error: can't run the C compiler '${CC:-cc}'."
+    exit 1
+fi
+cc_check=$(mktemp -d "${TMPDIR:-/tmp}/pseudo-configure.XXXXXX") || exit 1
+cat > "$cc_check/cc_check.c" <<EOF
+#if defined(__clang__)
+# if !defined(__has_include)
+#  error clang without __has_include
+# endif
+#elif !defined(__GNUC__) || __GNUC__ < 5
+# error GCC older than 5
+#endif
+int pseudo_cc_check;
+EOF
+if ! cc_check_out=$(${CC:-cc} -c -o "$cc_check/cc_check.o" "$cc_check/cc_check.c" 2>&1); then
+    echo >&2 "Error: pseudo requires GCC 5 or later (${CC:-cc} reports version $cc_version)."
+    echo >&2 "$cc_check_out"
+    rm -rf "$cc_check"
+    exit 1
+fi
+rm -rf "$cc_check"
+
 if [ ! -f "${opt_sqlite}/include/sqlite3.h" ]; then
     echo >&2 "SQLite3 headers not found in at ${opt_sqlite}/include/sqlite3.h. Please check that SQLite3 and SQLite3 headers are installed."
     exit 1