openssl: INSTALL_PREFIX renamed to DESTDIR, environ ignored

Because there's no way anymore to pass the install prefix via an environment
variable, a new `make-install-extra` option (empty by default) is added so
that `make install` command line can be extended with `DESTDIR=...`.

/reviewed-on !524

component/openssl/buildout.cfg

@@ -21,8 +21,9 @@ url = https://www.openssl.org/source/openssl-1.1.0j.tar.gz
 md5sum = b4ca5b78ae6ae79da80790b30dbedbdc
 location = @@LOCATION@@
 # 'prefix' option to override --openssldir/--prefix (which is useful
-# when combined with INSTALL_PREFIX). Used by slapos.package.git/obs
+# when combined with DESTDIR). Used by slapos.package.git/obs
 prefix = ${:location}
+make-install-extra =
 certs = ${:location}/etc/ssl/certs
 configure-command = ./config
 configure-options =
@@ -37,7 +38,7 @@ configure-options =
 make-options =
   SHARED_LDFLAGS='-Wl,-rpath=${:location}/lib -Wl,-rpath=${zlib:location}/lib'
 make-targets =
-  -j1 install_sw install_ssldirs &&
+  -j1 install_sw install_ssldirs ${:make-install-extra} &&
   rm -f ${:certs}/* &&
   for i in ${ca-certificates:location}/certs/*/*.crt; do
     ln -sfv $i ${:certs}/`${:location}/bin/openssl x509 -hash -noout -in $i`.0