config: check for parallel(1), use it for cstyle

Before:
$ time make cstyle
real    0m23.118s
user    0m23.002s
sys     0m0.114s

After:
$ time make cstyle
real    0m4.577s
user    0m31.487s
sys     0m0.699s

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Issue #12899

Makefile.am

@@ -114,6 +114,11 @@ commitcheck:
 		${top_srcdir}/scripts/commitcheck.sh; \
 	fi
 
+if HAVE_PARALLEL
+cstyle_line = -print0 | parallel -X0 ${top_srcdir}/scripts/cstyle.pl -cpP {}
+else
+cstyle_line = -exec ${top_srcdir}/scripts/cstyle.pl -cpP {} +
+endif
 PHONY += cstyle
 cstyle:
 	@find ${top_srcdir} -name build -prune \
@@ -122,7 +127,7 @@ cstyle:
 		! -name 'opt_global.h' ! -name '*_if*.h' \
 		! -name 'zstd_compat_wrapper.h' \
 		! -path './module/zstd/lib/*' \
-		-exec ${top_srcdir}/scripts/cstyle.pl -cpP {} \+
+		$(cstyle_line)
 
 filter_executable = -exec test -x '{}' \; -print
 

config/always-parallel.m4

@@ -0,0 +1,8 @@
+dnl #
+dnl # Check if GNU parallel is available.
+dnl #
+AC_DEFUN([ZFS_AC_CONFIG_ALWAYS_PARALLEL], [
+	AC_CHECK_PROG([PARALLEL], [parallel], [yes])
+
+	AM_CONDITIONAL([HAVE_PARALLEL], [test "x$PARALLEL" = "xyes"])
+])

config/zfs-build.m4

@@ -226,6 +226,7 @@ AC_DEFUN([ZFS_AC_CONFIG_ALWAYS], [
 	ZFS_AC_CONFIG_ALWAYS_SED
 	ZFS_AC_CONFIG_ALWAYS_CPPCHECK
 	ZFS_AC_CONFIG_ALWAYS_SHELLCHECK
+	ZFS_AC_CONFIG_ALWAYS_PARALLEL
 ])
 
 AC_DEFUN([ZFS_AC_CONFIG], [