Missing /MDd switch for debug build in nmake.opt

[daniel112@…]

The optimized build has /MD as default and the debug build is missing a /MDd
(OPTFLAGS) in nmake.opt

[warmerdam]

I don't generally like using runtime library builds for debug purposes. 
I *think* I can get into problems if some code uses the non-debug runtime
libraries and some uses the debug runtime libraries. So for debug builds I 
normally just build with static C libraries (the default). 

Note that the OPTFLAGS values are not intended to be set in stone.  They may
be altered freely by a developer who knows what they are doing and what they
want. 

I am marking this as "WONTFIX", but please re-open it if you think I am 
missing something. 

[Even Rouault]

In 40242:

nmake.opt: use /MDd for OPTFLAGS for DEBUG=1 builds (fixes #7059, fixes #6384, fixes #4291, fixes #3346, fixes #1647, fixes #540)