Reorganized the extlib/ subdirectory

LibPNG and zlib have been moved into their own subdirectories, consistent
with the SpaceWare Input library

Makefile.msvc

@@ -6,7 +6,7 @@ HAVE_SPACEWARE_INPUT = 1
 DEFINES = /D_WIN32_WINNT=0x500 /DISOLATION_AWARE_ENABLED /D_WIN32_IE=0x500 /DWIN32_LEAN_AND_MEAN /DWIN32 /DPACKAGE_VERSION="\"2.1\""
 # Use the multi-threaded static libc because libpng and zlib do; not sure if anything bad
 # happens if those mix, but don't want to risk it.
-CXXFLAGS = /W3 /nologo /MT /D_DEBUG /D_CRT_SECURE_NO_DEPRECATE /D_CRT_SECURE_NO_WARNINGS /I. /Iextlib /Zi /EHs  # /O2
+CXXFLAGS = /W3 /nologo /MT /D_DEBUG /D_CRT_SECURE_NO_DEPRECATE /D_CRT_SECURE_NO_WARNINGS /I. /Iextlib /Iextlib\libpng /Iextlib\zlib /Zi /EHs  # /O2
 
 HEADERS = win32\freeze.h ui.h solvespace.h dsc.h sketch.h expr.h polygon.h srf\surface.h
 
@@ -65,7 +65,7 @@ SRFOBJS =  $(OBJDIR)\boolean.obj \
 RES = $(OBJDIR)\resource.res
 
 LIBS = user32.lib gdi32.lib comctl32.lib advapi32.lib shell32.lib opengl32.lib glu32.lib \
-       extlib\libpng.lib extlib\zlib.lib
+       extlib\libpng\libpng.lib extlib\zlib\zlib.lib
 
 PERL = perl