Add static build support

This commit allows for building statically linked 86Box binaries.
Statically linked 86Box binaries do not need additional DLL files, which
improves user-friendliness significantly.

src/win/Makefile.mingw

@@ -638,9 +638,15 @@ OBJ		+= $(EXOBJ)
 endif
 
 LIBS		:= -mwindows \
-		   -lopenal.dll \
 		   -lddraw -ldxguid -ld3d9 \
 		   -lcomctl32 -lwinmm
+
+ifeq ($(STATIC), y)
+LIBS            += -lopenal -lole32
+else
+LIBS            += -lopenal.dll
+endif
+
 ifeq ($(D2D), y)
 LIBS		+= $(D2DLIB)
 endif
@@ -667,6 +673,10 @@ ifeq ($(D3DX), y)
 LIBS    += -ld3dx9
 endif
 
+ifeq ($(STATIC), y)
+LIBS    += -static
+endif
+
 # Build module rules.
 ifeq ($(AUTODEP), y)
 %.o:		%.c