Separated Makefile into both the BSD and the GNU versions

BSD version (proxy to `GNUmakefile`) -> `BSDmakefile`
GNU version -> `GNUmakefile`

.github/workflows/build-linux.yaml

@@ -7,7 +7,8 @@ on:
       - 'include/**'
       - 'src/**'
       - 'vendor/**'
-      - 'Makefile'
+      - 'BSDmakefile'
+      - 'GNUmakefile'
       - 'config.mk'
   pull_request:
     branches: [master]
@@ -16,7 +17,8 @@ on:
       - 'include/**'
       - 'src/**'
       - 'vendor/**'
-      - 'Makefile'
+      - 'BSDmakefile'
+      - 'GNUmakefile'
       - 'config.mk'
   schedule:
     - cron: '0 0 * * 1'

BSDmakefile

@@ -0,0 +1,26 @@
+#
+# GNU Sparky --- A 5v5 character-based libre tactical shooter
+# Copyright (C) 2024 Wasym A. Alonso
+#
+# This file is part of Sparky.
+#
+# Sparky is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# Sparky is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with Sparky.  If not, see <http://www.gnu.org/licenses/>.
+#
+
+
+.PHONY: all
+
+all:
+	@which gmake >/dev/null 2>&1 || (echo "ERROR: *** GNU make is required for Sparky to build.  Stop." && exit 1)
+	@gmake ${.FLAGS} ${.TARGETS}