Control building of ossl_shim through Configure

Don't build ossl_shim by default. Switch it on through
enable-external-tests.

Reviewed-by: Richard Levitte <levitte@openssl.org>

Configure

@@ -362,6 +362,7 @@ my @disablables = (
     "egd",
     "engine",
     "err",
+    "external-tests",
     "filenames",
     "fuzz-libfuzzer",
     "fuzz-afl",
@@ -431,6 +432,7 @@ our %disabled = ( # "what"         => "comment"
 		  "crypto-mdebug-backtrace" => "default",
 		  "ec_nistp_64_gcc_128" => "default",
 		  "egd"                 => "default",
+		  "external-tests"	=> "default",
 		  "fuzz-libfuzzer"	=> "default",
 		  "fuzz-afl"		=> "default",
 		  "heartbeats"          => "default",

INSTALL

@@ -310,6 +310,12 @@
   no-err
                    Don't compile in any error strings.
 
+  enable-external-tests
+                   Enable building of integration with external test suites.
+                   This is a developer option and may not work on all platforms.
+                   The only supported external test suite at the current time is
+                   the BoringSSL test suite.
+
   no-filenames
                    Don't compile in filename and line number information (e.g.
                    for errors and memory allocation).

test/ossl_shim/build.info

@@ -1,4 +1,4 @@
-IF[{- defined $target{cxx} -}]
+IF[{- defined $target{cxx} && !$disabled{"external-tests"}-}]
   PROGRAMS_NO_INST=ossl_shim
   SOURCE[ossl_shim]=ossl_shim.cc async_bio.cc packeted_bio.cc test_config.cc
   INCLUDE[ossl_shim]=. include ../../include