Always build libcurl as shared library

[marcothaller]

Ignore BUILD_SHARED_LIBS and always build libcurl as shared library. We cache current value of BUILD_SHARED_LIBS, set BUILD_SHARED_LIBS ON when including curl.cmake and reset BUILD_SHARED_LIBS to it's previous value after including curl.cmake.

@MiKom I tried to set BUILD_SHARED_LIBS for the curl target individually, using set_target_properties in curl.cmake but didn't succeed. My current solution does not appear very elegant OTOHS it is very unambiguous due to it's verbosity. :)

[MiKom]

Building curl as static library would be beneficial for easier deployment of the real application. Perhaps we could do this override only when tests are being built? And add a comment that it's due to faking the curl library for automated tests.

And btw. msvc is still not too happy about it:

[build] libcurl-d_imp.lib(libcurl-d.dll) : error LNK2005: curl_easy_init already defined in tst_network_access_manager.obj [C:\kdab\rnd\mecaps\code\mecaps\build\src\network_access_manager\test_network_access_manager.vcxproj]
[build] libcurl-d_imp.lib(libcurl-d.dll) : error LNK2005: curl_easy_setopt already defined in tst_network_access_manager.obj [C:\kdab\rnd\mecaps\code\mecaps\build\src\network_access_manager\test_network_access_manager.vcxproj]
[build] libcurl-d_imp.lib(libcurl-d.dll) : error LNK2005: curl_easy_cleanup already defined in tst_network_access_manager.obj [C:\kdab\rnd\mecaps\code\mecaps\build\src\network_access_manager\test_network_access_manager.vcxproj]
[build] libcurl-d_imp.lib(libcurl-d.dll) : error LNK2005: curl_easy_getinfo already defined in tst_network_access_manager.obj [C:\kdab\rnd\mecaps\code\mecaps\build\src\network_access_manager\test_network_access_manager.vcxproj]
[build] libcurl-d_imp.lib(libcurl-d.dll) : error LNK2005: curl_global_init already defined in tst_network_access_manager.obj [C:\kdab\rnd\mecaps\code\mecaps\build\src\network_access_manager\test_network_access_manager.vcxproj]
[build] libcurl-d_imp.lib(libcurl-d.dll) : error LNK2005: curl_multi_init already defined in tst_network_access_manager.obj [C:\kdab\rnd\mecaps\code\mecaps\build\src\network_access_manager\test_network_access_manager.vcxproj]
[build] libcurl-d_imp.lib(libcurl-d.dll) : error LNK2005: curl_multi_add_handle already defined in tst_network_access_manager.obj [C:\kdab\rnd\mecaps\code\mecaps\build\src\network_access_manager\test_network_access_manager.vcxproj]
[build] libcurl-d_imp.lib(libcurl-d.dll) : error LNK2005: curl_multi_remove_handle already defined in tst_network_access_manager.obj [C:\kdab\rnd\mecaps\code\mecaps\build\src\network_access_manager\test_network_access_manager.vcxproj]
[build] libcurl-d_imp.lib(libcurl-d.dll) : error LNK2005: curl_multi_info_read already defined in tst_network_access_manager.obj [C:\kdab\rnd\mecaps\code\mecaps\build\src\network_access_manager\test_network_access_manager.vcxproj]
[build] libcurl-d_imp.lib(libcurl-d.dll) : error LNK2005: curl_multi_socket_action already defined in tst_network_access_manager.obj [C:\kdab\rnd\mecaps\code\mecaps\build\src\network_access_manager\test_network_access_manager.vcxproj]
[build] libcurl-d_imp.lib(libcurl-d.dll) : error LNK2005: curl_multi_setopt already defined in tst_network_access_manager.obj [C:\kdab\rnd\mecaps\code\mecaps\build\src\network_access_manager\test_network_access_manager.vcxproj]

But let's not yet fix it here. I hope I'll be able to sit down to fixing some windows issues later this week. Possibly we'll be able to get away with /FORCE:MULTIPLE linker flag. We'll see

[marcothaller]

Regarding the linker error using MSVC: that is indeed a bummer and sth I wouldn't have expected (at least from the conclusions I had drawn after my investigation).
I noticed that the linker does not throw an error and only throws these kind of warnings warning C4273: 'curl_global_init': inconsistent dll linkage on your latest GitHub actions windows build.
If I were to make a guess, I'd guess error LNK2005 occurred building on your local machine with the find_package() call in the first line of curl.cmake finding a shared libcurl on your machine and using this to link - even though you just build a static libcurl (at least I ran into this issue when testing the commit on linux).

[MiKom]

Hey, which build that succeeded you had in mind?

I took a stab at the problem again but couldn't make it work even with /FORCE:MULTIPLE. The network access manager library is always calling the original curl dll and not the mocked symbols. This approach probably won't fly on Windows.