CMake: handle _INCLUDE_DIR not leveraged at compile time

[jmckenna]

Background

• the issue affects those on Windows compiling their own libraries, but also pointing to their include folders separately, with CMake's xxx_INCLUDE_DIR setting for each of their dependent libraries
• since the 3.9.3 release those include dir settings have not been leveraged by CMake at compile time (meaning: all libs will be reported as found by CMake, but then at compile time, CMake will not find the include files)
  • many common packaging systems get around the CMake xxx_INCLUDE_DIR not finding include files by keeping all include files in the same directory (so something like the path of curl/curl.h can always be found by any other library's files)
    • most likely your test environment here uses Conda or vcpkg or some other system that does exactly that
• so you might have been noticing Windows users since the 4.9.3 release saying "hi I compile all my libraries myself but recently I have this compile error for netcdf-c of zlib.h - no such file or directory, or curl/curl.h, or mfhdf.h, etc
  • similar to [CMAKE] build with HDF5 with external zlib #3099

Environment

• Windows & Visual Studio compiler
• compiled all dependent external libraries myself/themselves
  • in my case: such as hdf4 & hdf5 (git master from today), curl, zlib, etc etc
• testing with today's netcdf-c git main

Error messages during compile

• with Curl enabled

C:\netcdf-c-git-jmckenna\libdispatch\ddispatch.c(28): fatal error C1083: Cannot open include file: 'curl/curl.h': No such file or directory

• with HDF4 enabled

C:\netcdf-c-git-jmckenna\libhdf4\hdf4file.c(14): fatal error C1083: Cannot open include file: 'mfhdf.h': No such file or directory

• with zlib enabled

C:\netcdf-c-git-jmckenna\plugins\H5Zdeflate.c(23): fatal error C1083: Cannot open include file: 'zlib.h': No such file or directory

Workarounds

• see this pull request for the workarounds
• this pull request as-is will fix the user reported compile errors
  • however, most likely there is a more elegant way to handle this
    • such as moving that target_include_directories statement from CMakeLists.txt from the top, down into some other if (NETCDF_ENABLE_xxx) section of that CMakeLists.txt)
      • I just don't know which plugins etc require these includes, so it will need input here from the netcdf-c maintainers

My goal here is to post my workaround, and hopefully help other packagers, and maybe, just maybe, get these changes included in the next netcdf-c release

Thanks again,

[jmckenna]

Important to stress again: CMake reports all libraries as found, the compile errors will happen not when you run the cmake command, but during the compile command (however you choose to generate your MSVC scripts).