Add license headers to the Makefiles

Makefile.am

+# Copyright (c) 2013-2024 MPI-M, Luis Kornblueh, Rahul Sinha and DWD, Florian Prill. All rights reserved.
+#
+# SPDX-License-Identifier: BSD-3-Clause
+#
 SUBDIRS = include src doc examples test
 ACLOCAL_AMFLAGS = -I m4
 

doc/Makefile.am

+# Copyright (c) 2013-2024 MPI-M, Luis Kornblueh, Rahul Sinha and DWD, Florian Prill. All rights reserved.
+#
+# SPDX-License-Identifier: BSD-3-Clause
+#
 @DX_RULES@
 
 .NOTPARALLEL:

examples/Makefile.am

+# Copyright (c) 2013-2024 MPI-M, Luis Kornblueh, Rahul Sinha and DWD, Florian Prill. All rights reserved.
+#
+# SPDX-License-Identifier: BSD-3-Clause
+#
 if ENABLE_EXAMPLES
 noinst_PROGRAMS =     \
     callback_test     \

include/Makefile.am

+# Copyright (c) 2013-2024 MPI-M, Luis Kornblueh, Rahul Sinha and DWD, Florian Prill. All rights reserved.
+#
+# SPDX-License-Identifier: BSD-3-Clause
+#
 include_HEADERS =             \
     mtime_calendar.h          \
     mtime_calendar360day.h    \

src/Makefile.am

+# Copyright (c) 2013-2024 MPI-M, Luis Kornblueh, Rahul Sinha and DWD, Florian Prill. All rights reserved.
+#
+# SPDX-License-Identifier: BSD-3-Clause
+#
 lib_LTLIBRARIES = libmtime.la
 
 AM_CPPFLAGS = -I$(top_srcdir)/include

test/Makefile.am

+# Copyright (c) 2013-2024 MPI-M, Luis Kornblueh, Rahul Sinha and DWD, Florian Prill. All rights reserved.
+#
+# SPDX-License-Identifier: BSD-3-Clause
+#
 EXTRA_LTLIBRARIES = libmtime_check.la
 check_PROGRAMS = test_runner
 TESTS = test_runner