CI/cirrus: add shared and static Windows release builds

[mback2k]

Azure Pipelines is currently being used for debug builds, let's try to also run some non-debug (release) builds and make use of underutilized Cirrus CI for that.

[mback2k]

@jay @MarcelRaad I would appreciate your help if I got the flags for the static build variants correctly. It worked for me in the past.

[MarcelRaad]

Is this manual setting of -DCURL_STATICLIB and -static really necessary? I don't think I've ever had to set that for curl itself. At least CURL_STATICLIB should be defined here:

curl/configure.ac

Line 348 in 47e169e

CPPFLAG_CURL_STATICLIB='-DCURL_STATICLIB'

Other than that, looks good to me.

[mback2k]

I don't think I've ever had to set that for curl itself. At least CURL_STATICLIB should be defined here:

I was following the guidance given here: https://curl.se/docs/install.html

Important static libcurl usage note
When building an application that uses the static libcurl library on Windows, you must add -DCURL_STATICLIB to your CFLAGS. Otherwise the linker will look for dynamic import symbols.

Would CPPFLAG be used by a plain c build (not using c++)?

[MarcelRaad]

When building an application that uses the static libcurl library on Windows, you must add -DCURL_STATICLIB to your CFLAGS. Otherwise the linker will look for dynamic import symbols.

That applies to external applications using libcurl. I don't think it applies to curl itself as the build system should take care of that.

Would CPPFLAG be used by a plain c build (not using c++)?

For both, yes. CPPFLAGS is for the preprocessor, CFLAGS for the C compiler, and CXXFLAGS for the C++ compiler.

[mback2k]

Relevant CI builds passed, merging.