dub.sdl: Add an ae:openssl/lib-1.0 subconfiguration

This allows using ae:openssl on systems where /usr/lib/libssl.so is
1.1 or newer.

To do so, run e.g.: dub build --override-config=ae:openssl/lib-1.0

At some point, this configuration will become the default.

This commit would also have added configurations to select the openssl
D library version, if not for a failure in Dub's design which
prohibits doing so: dlang/dub#1706

dub.sdl

@@ -88,7 +88,12 @@ subPackage {
 	dependency "openssl" version="~>1.1"
 	sourceFiles \
 		"net/ssl/openssl.d"
-	libs "ssl" "crypto" platform="posix"
+	configuration "lib-current" {
+		libs "ssl" "crypto" platform="posix"
+	}
+	configuration "lib-1.0" {
+		libs ":libssl.so.1.0.0" ":libcrypto.so.1.0.0" platform="posix"
+	}
 	libs "ssl" "eay" platform="windows-x86"
 	libs "ssleay32" "libeay32" platform="windows-x86_64"
 }