autotools: more robust RC condition

Previous solution assumed that when building a Windows target, `RC` tool
is available and auto-detected by autotools. Sometime this is not true.

Rename and extend the `OS_WINDOWS` conditional to also check if `RC` is
set. Effectively this patch replaces a hard error with just not building
the resources.

The correct fix is (possibly) to set the TRIPLET to a valid one (so that
auto-detection works correctly), and a workaround is to manually point
the `RC` envvar to a working copy of `windres`.

[^1]: https://tg.st/u/ad68a31462958a3ef0c801258bf5e8dde310d9fe062416d37d42f1e996af6bdb.txt

Follow-up to 6de7322

Ref: https://curl.se/mail/lib-2022-10/0049.html

Closes curl#9781

configure.ac

@@ -570,7 +570,8 @@ CURL_DARWIN_CFLAGS
 CURL_DARWIN_SYSTEMCONFIGURATION
 CURL_SUPPORTS_BUILTIN_AVAILABLE
 
-AM_CONDITIONAL([OS_WINDOWS], [test "$curl_cv_native_windows" = "yes"])
+AM_CONDITIONAL([HAVE_WINDRES],
+  test "$curl_cv_native_windows" = "yes" && test -n "${RC}")
 
 dnl ************************************************************
 dnl switch off particular protocols

lib/Makefile.am

@@ -109,7 +109,7 @@ endif
 if USE_CPPFLAG_CURL_STATICLIB
 libcurl_la_CPPFLAGS_EXTRA += -DCURL_STATICLIB
 else
-if OS_WINDOWS
+if HAVE_WINDRES
 libcurl_la_SOURCES += $(LIB_RCFILES)
 $(LIB_RCFILES): $(top_srcdir)/include/curl/curlver.h
 endif
@@ -154,7 +154,7 @@ tidy:
 optiontable:
 	perl optiontable.pl < $(top_srcdir)/include/curl/curl.h > easyoptions.c
 
-if OS_WINDOWS
+if HAVE_WINDRES
 # Warning is "normal": libtool:   error: ignoring unknown tag RC
 .rc.lo:
 	$(LIBTOOL) --tag=RC --mode=compile $(RC) -I$(top_srcdir)/include $(RCFLAGS) -i $< -o $@

src/Makefile.am

@@ -55,7 +55,7 @@ include Makefile.inc
 
 # CURL_FILES comes from Makefile.inc
 curl_SOURCES = $(CURL_FILES)
-if OS_WINDOWS
+if HAVE_WINDRES
 curl_SOURCES += $(CURL_RCFILES)
 $(CURL_RCFILES): tool_version.h
 endif
@@ -156,7 +156,7 @@ tidy:
 listhelp:
 	(cd $(top_srcdir)/docs/cmdline-opts && ./gen.pl listhelp *.d) > tool_listhelp.c
 
-if OS_WINDOWS
+if HAVE_WINDRES
 # Warning is "normal": libtool:   error: ignoring unknown tag RC
 .rc.o:
 	$(LIBTOOL) --tag=RC --mode=compile $(RC) -I$(top_srcdir)/include -DCURL_EMBED_MANIFEST $(RCFLAGS) -i $< -o $@