Rewrite autoconf libcurl detection

[hmelder]

• Bump minimum supported version to 7.66.0 and remove old checks for quirks
• Use pkg-config instead of curl-config, as curl-config is not installed when targeting Windows.
• Remove gnutls warning, as no loss of functionality is observed.

[rfm]

I think there are multiple problems with this patch breaking things.
The use of pkg-config is a last resort because it is not be available on some systems.
If we don't have curl-config to tell us about curl, we could try pkg-config as a fallback.

[fredkiefer]

Shouldn't you be using $PKG_CONFIG here as well (and in line 3631)?

[hmelder]

You are right