Fix rc compiler redefinition warnings (nmake)

Fix rc compiler redefinition warnings (nmake)

- Surround code in win32/rrd_config.h that causes warnings by rc with
  #ifndef RC_INVOKED and #endif. Remark: _MSC_VER is unknown to rc
- Fixes the following warnings for .rc files, e.g.:
  rc /nologo /I./src /fo./win32/rrdtool.res ./win32\rrdtool.rc
  ./win32\rrd_config.h(130) : warning RC4005: 'isinf' : redefinition
  ./win32\rrd_config.h(131) : warning RC4005: 'isnan' : redefinition
- This concerns: librrd-4.rc, rrdtool.rc, rrdupdate.rc, rrdcgi.rc,
  when using the resource compiler (rc) via nmake under Windows