Curl is hard coded to off for all Windows

[kmilos]

From the readme: -DUSE_CURL=OFF (default is ON, but unavailable in MS Visual Studio)

cfitsio/CMakeLists.txt

Line 75 in 3b6fc6f

IF(NOT WIN32)

However, there should be no problem using it w/ MinGW, which is also covered by the CMake WIN32 define.

Did you perhaps mean if(NOT MSVC) here instead?

Same goes for the USE_BZIP2 gating as well - there's no real reason it wouldn't work w/ MinGW...

[kmilos]

Ah, this fails for another reason, it's not just MSVC only... Looks like there is symbol clash also w/ MinGW:

FAILED: CMakeFiles/cfitsio.dir/cfileio.c.obj 
  D:\M\msys64\clang64\bin\clang.exe -DCFITSIO_HAVE_CURL -DHAVE_FTRUNCATE -DHAVE_UNISTD_H -D_REENTRANT -IC:/_/B/src/cfitsio-4.5.0 -IC:/_/B/src/static-CLANG64 -march=nocona -msahf -mtune=generic -O2 -pipe -Wp,-D_FORTIFY_SOURCE=2 -fstack-protector-strong -Wp,-D__USE_MINGW_ANSI_STDIO=1 -D_LARGEFILE_SOURCE=ON -D_FILE_OFFSET_BITS=64 -O3 -DNDEBUG -msse2 -MD -MT CMakeFiles/cfitsio.dir/cfileio.c.obj -MF CMakeFiles\cfitsio.dir\cfileio.c.obj.d -o CMakeFiles/cfitsio.dir/cfileio.c.obj -c C:/_/B/src/cfitsio-4.5.0/cfileio.c
  In file included from C:/_/B/src/cfitsio-4.5.0/cfileio.c:16:
  In file included from D:/M/msys64/clang64/include/curl/curl.h:81:
  In file included from D:/M/msys64/clang64/include/winsock2.h:23:
  In file included from D:/M/msys64/clang64/include/windows.h:69:
  In file included from D:/M/msys64/clang64/include/windef.h:9:
  In file included from D:/M/msys64/clang64/include/minwindef.h:163:
  D:/M/msys64/clang64/include/winnt.h:384:25: error: expected identifier or '('
  
    384 |   typedef unsigned char TBYTE, *PTBYTE;
  
        |                         ^
  
  C:/_/B/src/cfitsio-4.5.0/fitsio.h:223:22: note: expanded from macro 'TBYTE'
  
    223 | #define TBYTE        11
  
        |                      ^
  
  1 error generated.

May I suggest simply renaming your TBYTE to e.g. T_BYTE everywhere?