cmake: support building some complicated examples, build them in CI

Build these examples when the necessary dependencies are present:
- cacertinmem, usercertinmem (OpenSSL/fork)
- multi-uv (libuv)
- multithread, threaded-ssl (pthread)
- sessioninfo (GnuTLS)

Indicate the necessary dependency via a `Required:` comment placed in
the source file. A single dependency per source is supported as of now.
The name of the dependency should match the variable used within
the cmake scripts, which in turn matches the macro used in the config
header. E.g. for GnuTLS it's `USE_GNUTLS`.

Also:
- GHA/macos: build examples in two job to test GnuTLS and pthread ones.
- GHA/linux: enable libuv to test it with examples.

Follow-up to 6bb7714032 #18914
Closes #18909

docs/examples/Makefile.am

@@ -24,7 +24,8 @@
 
 AUTOMAKE_OPTIONS = foreign nostdinc
 
-EXTRA_DIST = CMakeLists.txt .checksrc README.md Makefile.example $(COMPLICATED_EXAMPLES)
+EXTRA_DIST = CMakeLists.txt .checksrc README.md Makefile.example \
+  $(COMPLICATED_MAY_BUILD) $(COMPLICATED_EXAMPLES)
 
 # Specify our include paths here, and do it relative to $(top_srcdir) and
 # $(top_builddir), to ensure that these paths which belong to the library
@@ -53,7 +54,7 @@ LDADD = $(top_builddir)/lib/libcurl.la
 # This might hold -Werror
 CFLAGS += @CURL_CFLAG_EXTRAS@
 
-# Get check_PROGRAMS variable
+# Get check_PROGRAMS, COMPLICATED_MAY_BUILD, COMPLICATED_EXAMPLES variables
 include Makefile.inc
 
 all: $(check_PROGRAMS)