Makefile changes:

1. If OSD is not defined, we look for the built-in Windows
environment variable 'OS' and if it is set to 'Windows_NT',
we assume OSD=windows; otherwise we assume OSD=sdl.

2. If TARGETOS is not defined, we look for the same variable
and set TARGETOS=win32 if it is set to 'Windows_NT'.

3. Windows SDL builds have an 'sdl' prefix on the executable
so they can live side-by-side.

The upshot of this is that on Windows systems:

make -> standard Windows build producing mame.exe
make OSD=sdl -> SDL Windows builds producing sdlmame.exe

On non-Windows systems:

make -> SDL build by default (don't need to say OSD=sdl)

makefile

@@ -40,7 +40,12 @@ endif
 #-------------------------------------------------
 
 ifndef OSD
+ifeq ($(OS),Windows_NT)
 OSD = windows
+TARGETOS = win32
+else
+OSD = sdl
+endif
 endif
 
 ifndef CROSS_BUILD_OSD
@@ -56,7 +61,7 @@ endif
 #-------------------------------------------------
 
 ifndef TARGETOS
-ifeq ($(OSD),windows)
+ifeq ($(OS),Windows_NT)
 TARGETOS = win32
 else
 TARGETOS = unix
@@ -113,6 +118,7 @@ endif
 # MACOSX_USE_LIBSDL = 1
 
 
+
 #-------------------------------------------------
 # specify build options; see each option below 
 # for details
@@ -205,6 +211,18 @@ RM = @rm -f
 # form the name of the executable
 #-------------------------------------------------
 
+# reset all internal prefixes/suffixes
+PREFIXSDL =
+SUFFIX64 =
+SUFFIXDEBUG =
+
+# Windows SDL builds get an SDL prefix
+ifeq ($(OSD),sdl)
+ifeq ($(TARGETOS),win32)
+PREFIXSDL = sdl
+endif
+endif
+
 # 64-bit builds get a '64' suffix
 ifdef PTR64
 SUFFIX64 = 64
@@ -224,7 +242,7 @@ NAME = $(TARGET)$(SUBTARGET)
 endif
 
 # fullname is prefix+name+suffix+suffix64+suffixdebug
-FULLNAME = $(PREFIX)$(NAME)$(SUFFIX)$(SUFFIX64)$(SUFFIXDEBUG)
+FULLNAME = $(PREFIX)$(PREFIXSDL)$(NAME)$(SUFFIX)$(SUFFIX64)$(SUFFIXDEBUG)
 
 # add an EXE suffix to get the final emulator name
 EMULATOR = $(FULLNAME)$(EXE)