Makefile: automatically detect system to compile for, if no system was specified

This should work for mingw and linux so far.

(cherry picked from commit f92026f9)

src/Makefile

@@ -81,6 +81,57 @@
 #
 #############################################################################
 
+ALL_SYSTEMS=\
+	PANDORA\
+	LINUX64\
+	MINGW64\
+	HAIKU\
+	DUMMY\
+	DJGPPDOS\
+	MINGW\
+	UNIX\
+	LINUX\
+	SOLARIS\
+	FREEBSD\
+	MACOSX\
+	SDL\
+
+# check for user specified system
+ifeq (,$(filter $(ALL_SYSTEMS),$(.VARIABLES)))
+ifeq ($(OS),Windows_NT) # all windows are Windows_NT...
+
+ $(info Detected a Windows system, compiling for 32-bit MinGW SDL2...)
+
+ # go for a 32-bit sdl mingw exe by default
+ MINGW=1
+ SDL=1
+
+else # if you on the *nix
+
+ system:=$(shell uname -s)
+
+ ifeq ($(system),Linux)
+ new_system=LINUX
+ else
+
+ $(error \
+	 Could not automatically detect your system,\
+	 try specifying a system manually)
+
+ endif
+
+ ifeq ($(shell getconf LONG_BIT),64)
+ system+=64-bit
+ new_system:=$(new_system)64
+ endif
+
+ $(info Detected $(system) ($(new_system))...)
+ $(new_system)=1
+
+endif
+endif
+
+
 # SRB2 data files
 D_DIR?=../bin/Resources
 D_FILES=$(D_DIR)/srb2.srb \