BUILD: makefile: rework 51D to split v3/v4

There are multiple options for 51DEGREES, v3/v4, threading or not,
pattern/trie for v3, vhash for v4, use of libatomic, etc. While the
current rules deal with all of that correctly, it's too difficult to
focus on one version because the two are interleaved for every single
option. Let's just split them into two independent blocks. This removes
some if/endif, and makes the lecture much more straightforward.

Makefile

@@ -660,46 +660,41 @@ ifneq ($(USE_DEVICEATLAS),)
   OPTIONS_CFLAGS += $(if $(DEVICEATLAS_INC),-I$(DEVICEATLAS_INC))
 endif
 
-ifneq ($(USE_51DEGREES),)
-  ifneq ($(USE_51DEGREES_V4),)
+# Use 51DEGREES_SRC and possibly 51DEGREES_INC and 51DEGREES_LIB to force path
+# to 51degrees v3/v4 headers and libraries if needed.
+51DEGREES_INC = $(51DEGREES_SRC)
+51DEGREES_LIB = $(51DEGREES_SRC)
+
+ifneq ($(USE_51DEGREES_V4),)  # v4 here
+  ifneq ($(USE_51DEGREES),)
     $(error cannot compile both 51Degrees V3 and V4 engine support)
   endif
-endif
-
-ifneq ($(USE_51DEGREES)$(USE_51DEGREES_V4),)
-  # Use 51DEGREES_SRC and possibly 51DEGREES_INC and 51DEGREES_LIB to force path
-  # to 51degrees headers and libraries if needed.
-  51DEGREES_INC = $(51DEGREES_SRC)
-  51DEGREES_LIB = $(51DEGREES_SRC)
-  ifneq ($(USE_51DEGREES_V4),)
-    _51DEGREES_SRC   = $(shell find $(51DEGREES_LIB) -maxdepth 2 -name '*.c')
-    OPTIONS_OBJS    += $(_51DEGREES_SRC:%.c=%.o)
-  else
-    OPTIONS_OBJS    += $(51DEGREES_LIB)/../cityhash/city.o
-    OPTIONS_OBJS    += $(51DEGREES_LIB)/51Degrees.o
+  _51DEGREES_SRC   = $(shell find $(51DEGREES_LIB) -maxdepth 2 -name '*.c')
+  OPTIONS_OBJS    += $(_51DEGREES_SRC:%.c=%.o)
+  OPTIONS_CFLAGS  += -DUSE_51DEGREES_V4
+  ifeq ($(USE_THREAD),)
+    OPTIONS_CFLAGS  += -DFIFTYONEDEGREES_NO_THREADING -DFIFTYONE_DEGREES_NO_THREADING
   endif
   OPTIONS_OBJS    += addons/51degrees/51d.o
   OPTIONS_CFLAGS  += $(if $(51DEGREES_INC),-I$(51DEGREES_INC))
-  ifneq ($(USE_51DEGREES_V4),)
-    OPTIONS_CFLAGS  += -DUSE_51DEGREES_V4
-  endif
+  OPTIONS_LDFLAGS += $(if $(51DEGREES_LIB),-L$(51DEGREES_LIB))
+  USE_ATOMIC       = implicit
+  USE_MATH         = implicit
+endif # USE_51DEGREES_V4
+
+ifneq ($(USE_51DEGREES),) # v3 here
+  OPTIONS_OBJS    += $(51DEGREES_LIB)/../cityhash/city.o
+  OPTIONS_OBJS    += $(51DEGREES_LIB)/51Degrees.o
   ifeq ($(USE_THREAD),)
     OPTIONS_CFLAGS  += -DFIFTYONEDEGREES_NO_THREADING
-    ifneq ($(USE_51DEGREES_V4),)
-      OPTIONS_CFLAGS  += -DFIFTYONE_DEGREES_NO_THREADING
-    endif
-    else
-      ifeq ($(USE_51DEGREES_V4),)
-      OPTIONS_OBJS    += $(51DEGREES_LIB)/../threading.o
-    endif
+  else
+    OPTIONS_OBJS    += $(51DEGREES_LIB)/../threading.o
   endif
-
+  OPTIONS_OBJS    += addons/51degrees/51d.o
+  OPTIONS_CFLAGS  += $(if $(51DEGREES_INC),-I$(51DEGREES_INC))
   OPTIONS_LDFLAGS += $(if $(51DEGREES_LIB),-L$(51DEGREES_LIB))
   USE_MATH         = implicit
-  ifneq ($(USE_51DEGREES_V4),)
-    USE_LIBATOMIC    = implicit
-  endif
-endif
+endif # USE_51DEGREES
 
 ifneq ($(USE_WURFL),)
   # Use WURFL_SRC and possibly WURFL_INC and WURFL_LIB to force path