Lazy load libcurl, allowing for an SSL/TLS backend-specific libcurl (#4410)

As per
#4350 (comment),
the major block for upgrading Git for Windows' OpenSSL from v1.1 to v3
is the tricky part where such an upgrade would break `git fetch`/`git
clone` and `git push` because the libcurl depends on the OpenSSL DLL,
and the major version bump will _change_ the file name of said DLL.

To overcome that, the plan is to build libcurl flavors for each
supported SSL/TLS backend, aligning with the way MSYS2 builds libcurl,
then switch Git for Windows' SDK to the Secure Channel-flavored libcurl,
and teach Git to look for the specific flavor of libcurl corresponding
to the `http.sslBackend` setting (if that was configured).

Here is the PR to teach Git that trick.

Author: dscho

Makefile

@@ -464,6 +464,11 @@ include shared.mak
 #
 #     CURL_LDFLAGS=-lcurl
 #
+# Define LAZYLOAD_LIBCURL to dynamically load the libcurl; This can be useful
+# if Multiple libcurl versions exist (with different file names) that link to
+# various SSL/TLS backends, to support the `http.sslBackend` runtime switch in
+# such a scenario.
+#
 # === Optional library: libpcre2 ===
 #
 # Define USE_LIBPCRE if you have and want to use libpcre. Various
@@ -1612,10 +1617,23 @@ else
 		CURL_LIBCURL =
 	endif
 
-	ifndef CURL_LDFLAGS
-		CURL_LDFLAGS = $(eval CURL_LDFLAGS := $$(shell $$(CURL_CONFIG) --libs))$(CURL_LDFLAGS)
+	ifdef LAZYLOAD_LIBCURL
+		LAZYLOAD_LIBCURL_OBJ = compat/lazyload-curl.o
+		OBJECTS += $(LAZYLOAD_LIBCURL_OBJ)
+		# The `CURL_STATICLIB` constant must be defined to avoid seeing the functions
+		# declared as DLL imports
+		CURL_CFLAGS = -DCURL_STATICLIB
+ifneq ($(uname_S),MINGW)
+ifneq ($(uname_S),Windows)
+		CURL_LIBCURL = -ldl
+endif
+endif
+	else
+		ifndef CURL_LDFLAGS
+			CURL_LDFLAGS = $(eval CURL_LDFLAGS := $$(shell $$(CURL_CONFIG) --libs))$(CURL_LDFLAGS)
+		endif
+		CURL_LIBCURL += $(CURL_LDFLAGS)
 	endif
-	CURL_LIBCURL += $(CURL_LDFLAGS)
 
 	ifndef CURL_CFLAGS
 		CURL_CFLAGS = $(eval CURL_CFLAGS := $$(shell $$(CURL_CONFIG) --cflags))$(CURL_CFLAGS)
@@ -1636,7 +1654,7 @@ else
 	endif
 	ifdef USE_CURL_FOR_IMAP_SEND
 		BASIC_CFLAGS += -DUSE_CURL_FOR_IMAP_SEND
-		IMAP_SEND_BUILDDEPS = http.o
+		IMAP_SEND_BUILDDEPS = http.o $(LAZYLOAD_LIBCURL_OBJ)
 		IMAP_SEND_LDFLAGS += $(CURL_LIBCURL)
 	endif
 	ifndef NO_EXPAT
@@ -2843,10 +2861,10 @@ git-imap-send$X: imap-send.o $(IMAP_SEND_BUILDDEPS) GIT-LDFLAGS $(GITLIBS)
 	$(QUIET_LINK)$(CC) $(ALL_CFLAGS) -o $@ $(ALL_LDFLAGS) $(filter %.o,$^) \
 		$(IMAP_SEND_LDFLAGS) $(LIBS)
 
-git-http-fetch$X: http.o http-walker.o http-fetch.o GIT-LDFLAGS $(GITLIBS)
+git-http-fetch$X: http.o http-walker.o http-fetch.o $(LAZYLOAD_LIBCURL_OBJ) GIT-LDFLAGS $(GITLIBS)
 	$(QUIET_LINK)$(CC) $(ALL_CFLAGS) -o $@ $(ALL_LDFLAGS) $(filter %.o,$^) \
 		$(CURL_LIBCURL) $(LIBS)
-git-http-push$X: http.o http-push.o GIT-LDFLAGS $(GITLIBS)
+git-http-push$X: http.o http-push.o $(LAZYLOAD_LIBCURL_OBJ) GIT-LDFLAGS $(GITLIBS)
 	$(QUIET_LINK)$(CC) $(ALL_CFLAGS) -o $@ $(ALL_LDFLAGS) $(filter %.o,$^) \
 		$(CURL_LIBCURL) $(EXPAT_LIBEXPAT) $(LIBS)
 
@@ -2856,7 +2874,7 @@ $(REMOTE_CURL_ALIASES): $(REMOTE_CURL_PRIMARY)
 	ln -s $< $@ 2>/dev/null || \
 	cp $< $@
 
-$(REMOTE_CURL_PRIMARY): remote-curl.o http.o http-walker.o GIT-LDFLAGS $(GITLIBS)
+$(REMOTE_CURL_PRIMARY): remote-curl.o http.o http-walker.o $(LAZYLOAD_LIBCURL_OBJ) GIT-LDFLAGS $(GITLIBS)
 	$(QUIET_LINK)$(CC) $(ALL_CFLAGS) -o $@ $(ALL_LDFLAGS) $(filter %.o,$^) \
 		$(CURL_LIBCURL) $(EXPAT_LIBEXPAT) $(LIBS)