Document how to force linking with static libraries

Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1889)

Configurations/README

@@ -339,6 +339,17 @@ source as well.  However, the files given through SOURCE are expected
 to be located in the source tree while files given through DEPEND are
 expected to be located in the build tree)
 
+It's also possible to depend on static libraries explicitely:
+
+    DEPEND[foo]=libsomething.a
+    DEPEND[libbar]=libsomethingelse.a
+
+This should be rarely used, and care should be taken to make sure it's
+only used when supported.  For example, native Windows build doesn't
+support build static libraries and DLLs at the same time, so using
+static libraries on Windows can only be done when configured
+'no-shared'.
+
 For some libraries, we maintain files with public symbols and their
 slot in a transfer vector (important on some platforms).  It can be
 declared like this:

Configurations/README.design

@@ -133,7 +133,7 @@ library 'libssl' is built from the source file 'ssl/tls.c'.
 
     ENGINES_NO_INST=ossltest
     SOURCE[ossltest]=e_ossltest.c
-    DEPEND[ossltest]=../libcrypto
+    DEPEND[ossltest]=../libcrypto.a
     INCLUDE[ossltest]=../include
 
 This is the build.info file in 'engines/', telling us that two engines
@@ -142,8 +142,9 @@ dasync's source is 'engines/e_dasync.c' and ossltest's source is
 'engines/e_ossltest.c' and that the include directory 'include/' may
 be used when building anything that will be part of these engines.
 Also, both engines depend on the library 'libcrypto' to function
-properly.  Finally, only dasync is being installed, as ossltest is
-only for internal testing.
+properly.  ossltest is explicitly linked with the static variant of
+the library 'libcrypto'.  Finally, only dasync is being installed, as
+ossltest is only for internal testing.
 
 When Configure digests these build.info files, the accumulated
 information comes down to this:
@@ -170,7 +171,7 @@ information comes down to this:
 
     ENGINES_NO_INST=engines/ossltest
     SOURCE[engines/ossltest]=engines/e_ossltest.c
-    DEPEND[engines/ossltest]=libcrypto
+    DEPEND[engines/ossltest]=libcrypto.a
     INCLUDE[engines/ossltest]=include
     
     GENERATE[crypto/buildinf.h]=util/mkbuildinf.pl "$(CC) $(CFLAGS)" "$(PLATFORM)"
@@ -281,10 +282,14 @@ section above would be digested into a %unified_info table:
                     [
                         "crypto/buildinf.h",
                     ],
-                "engines/ossltest" =>
+                "engines/dasync" =>
                     [
                         "libcrypto",
                     ],
+                "engines/ossltest" =>
+                    [
+                        "libcrypto.a",
+                    ],
                 "libssl" =>
                     [
                         "libcrypto",
@@ -396,6 +401,14 @@ section above would be digested into a %unified_info table:
                     [
                         "crypto/evp.c",
                     ],
+                "engines/e_dasync.o" =>
+                    [
+                        "engines/e_dasync.c",
+                    ],
+                "engines/dasync" =>
+                    [
+                        "engines/e_dasync.o",
+                    ],
                 "engines/e_ossltest.o" =>
                     [
                         "engines/e_ossltest.c",