Blind attempt at fixing the non-MSVC Windows builds

Apparently, they need -DBUILDING_DLL for the Assert() declarations to
work correctly.

src/makefiles/Makefile.cygwin

@@ -25,6 +25,10 @@ endif
 endif
 endif
 
+ifneq (,$(findstring src/common,$(subdir)))
+override CPPFLAGS+= -DBUILDING_DLL
+endif
+
 ifneq (,$(findstring timezone,$(subdir)))
 override CPPFLAGS+= -DBUILDING_DLL
 endif

src/makefiles/Makefile.win32

@@ -23,6 +23,10 @@ endif
 endif
 endif
 
+ifneq (,$(findstring src/common,$(subdir)))
+override CPPFLAGS+= -DBUILDING_DLL
+endif
+
 ifneq (,$(findstring timezone,$(subdir)))
 override CPPFLAGS+= -DBUILDING_DLL
 endif