Generate doc/build.info with 'make update' rather than on the fly

doc/build.info was essentially generated on the fly while running
Configure, something that takes a huge amount of time on slower file
systems (such as Windows).

Instead, we generate it with 'make update', saving the user from
having to wait for too long, at the small price for developers to have
to run 'make update' whenever they write a new manual file.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/14269)

Configurations/unix-Makefile.tmpl

@@ -1022,11 +1022,15 @@ uninstall_html_docs:
 
 # Developer targets (note: these are only available on Unix) #########
 
-update: generate errors ordinals
+# It's important that generate_buildinfo comes after ordinals, as ordinals
+# is sensitive to build.info changes.
+update: generate errors ordinals generate_buildinfo
 
 generate: generate_apps generate_crypto_bn generate_crypto_objects \
           generate_crypto_conf generate_crypto_asn1 generate_fuzz_oids
 
+generate_buildinfo: generate_doc_buildinfo
+
 .PHONY: doc-nits cmd-nits md-nits
 doc-nits: build_generated_pods
 	$(PERL) $(SRCDIR)/util/find-doc-nits -n -l -e
@@ -1089,6 +1093,11 @@ generate_fuzz_oids:
 				crypto/objects/obj_dat.h \
 				> fuzz/oids.txt )
 
+generate_doc_buildinfo:
+	( $(PERL) -I$(BLDDIR) -Mconfigdata \
+                $(SRCDIR)/util/dofile.pl -o Makefile \
+                        $(SRCDIR)/doc/build.info.in > $(SRCDIR)/doc/build.info )
+
 # Set to -force to force a rebuild
 ERROR_REBUILD=
 errors:

doc/build.info.in

@@ -4,6 +4,8 @@ SUBDIRS = man1
  use File::Spec::Functions qw(:DEFAULT abs2rel rel2abs);
  use File::Basename;
 
+ my $sourcedir = catdir($config{sourcedir}, 'doc');
+
  foreach my $section ((1, 3, 5, 7)) {
      my @htmlfiles = ();
      my @manfiles = ();