autotools: simplify configuration in tests, examples

- GHA/windows: make a mingw autotools build static only.
- GHA/windows: fix a CI script issue with the build above.
- src: fix to pass `LIBCURL_PC_LIBS_PRIVATE` instead of `LINKFLAGS`.
  This makes the libs propagate to tunits, making the local hack there
  unnecessary. `LINKFLAGS` had this single use in the repo, and it was
  empty in local tests.
- tests: drop passing redundant `LIBCURL_PC_LDFLAGS_PRIVATE`.
- tests: drop redundant target name from config variables.
- examples, tests/client: drop `LIBDIR` temp variables with single uses.
- examples, tests: formatting to sync `Makefile.am` scripts with each
  other.

Closes #17661

docs/examples/Makefile.am

@@ -36,23 +36,20 @@ EXTRA_DIST = README.md Makefile.example CMakeLists.txt \
 
 AM_CPPFLAGS = -I$(top_srcdir)/include
 
-LIBDIR = $(top_builddir)/lib
-
-# Avoid libcurl obsolete stuff
-AM_CPPFLAGS += -DCURL_NO_OLDIES
-
 if USE_CPPFLAG_CURL_STATICLIB
 AM_CPPFLAGS += -DCURL_STATICLIB
 endif
 if DOING_NATIVE_WINDOWS
 AM_CPPFLAGS += -DWIN32_LEAN_AND_MEAN
 endif
+# Avoid libcurl obsolete stuff
+AM_CPPFLAGS += -DCURL_NO_OLDIES
 
 # Prevent LIBS from being used for all link targets
 LIBS = $(BLANK_AT_MAKETIME)
 
 # Dependencies
-LDADD = $(LIBDIR)/libcurl.la @LIBCURL_PC_LIBS_PRIVATE@
+LDADD = $(top_builddir)/lib/libcurl.la
 
 # This might hold -Werror
 CFLAGS += @CURL_CFLAG_EXTRAS@