infra/pkg-perl: add possibility to pass extra env at configure time

Some perl packages may use environment variables as a hint to know how
to be configured.

That's for example the case for perl-net-ssleay that uses
OPENSSL_PREFIX, if it is set in the environment, as the prefix to
openssl.

Add a new variable that packages can set if they need extra environment
variables. Update the manual accordingly.

Signed-off-by: Francois Perrad <francois.perrad@gadz.org>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
[yann.morin.1998@free.fr: enhance the commit log]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
package/perl-net-ssleay: fix buildsystem for cross-compilation

The buildsystem for perl-net-ssleay is totally brain-damaged: it tries
to execute the openssl program. It uses that to check the openssl
version, detect the prefix to openssl, and set the include and library
search paths.

Of course, it does not handle the destdir idiom, so it ends up finding
the host system's openssl (not even ours!), so it sets search paths to
/usr/inlcude and /usr/lib and /lib.

Well, guess what? It does not work.

Fix that by:
  - passing the prefix to openssl in the environment, so it does not go
    hunting for the host-system openssl;
  - not checking the version string, since we can't run, and Buildroot
    has the correct version anyway;
  - not setting any -I or -L paths since our compiler/linker already
    know where to look for for includes and libs.

Fixes:
    http://autobuild.buildroot.net/results/135/135867ef85535863e3647cc5fb82accb6f77612c/

Signed-off-by: Francois Perrad <francois.perrad@gadz.org>
[me: enhance commit log, add patch to avoid running openssl and setting
 wrong include and library search paths]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
package/perl-net-ssleay: not available for static link

Perl extensions are loaded at runtime with dlopen(), so it does not
make sense to even build extensions that are written in C.

perl-net-ssleay is (partly) written in C, so it is not available when
doing a static build.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Francois Perrad <francois.perrad@gadz.org>
Cc: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
package/perl-xml-libxml: not available for static link

Perl extensions are loaded at runtime with dlopen(), so it does not
make sense to even build extensions that are written in C.

perl-xml-libxml is written in C, so it is not available when doing a
static build.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Francois Perrad <francois.perrad@gadz.org>
Cc: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>