OpenSSL 1.1 breaks configure and more

Hi,

as reported by Debian's OpenSSL maintainers, PostgreSQL is failing to
build against a snapshot of the upcoming 1.1.0 version. The report was
for 9.5.3, but I can reproduce it in HEAD as well:

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=828510
> OpenSSL 1.1.0 is about to released. During a rebuild of all packages using
> OpenSSL this package fail to build. A log of that build can be found at:
> https://breakpoint.cc/openssl-1.1-rebuild-2016-05-29/Attempted/postgresql-9.5_9.5.3-1_amd64-20160529-1510
>
> On https://wiki.openssl.org/index.php/1.1_API_Changes you can see various of the
> reasons why it might fail. There are also updated man pages at
> https://www.openssl.org/docs/manmaster/ that should contain useful information.
>
> There is a libssl-dev package available in experimental that contains a recent
> snapshot, I suggest you try building against that to see if everything works.

$ ./configure --with-openssl
checking for CRYPTO_new_ex_data in -lcrypto... yes
checking for SSL_library_init in -lssl... no
configure: error: library 'ssl' is required for OpenSSL

I can get one step further by tweaking configure.in and running
autoreconf, but then compilation fails further down:

- AC_CHECK_LIB(ssl, SSL_library_init, [], [AC_MSG_ERROR([library 'ssl' is required for OpenSSL])])
+ AC_CHECK_LIB([ssl], [SSL_library_init])

make -C common all
make[4]: Verzeichnis „/home/cbe/projects/postgresql/pg/master/src/backend/access/common“ wird betreten
gcc -Wall -Wmissing-prototypes -Wpointer-arith -Wdeclaration-after-statement -Wendif-labels -Wmissing-format-attribute -Wformat-security -fno-strict-aliasing -fwrapv -fexcess-precision=standard -O2 -I../../../../src/include -D_GNU_SOURCE -c -o printtup.o printtup.c
In file included from ../../../../src/include/libpq/libpq-be.h:25:0,
from ../../../../src/include/libpq/libpq.h:21,
from printtup.c:19:
/usr/include/openssl/ssl.h:1740:26: error: expected identifier or ‘(’ before numeric constant
__owur const COMP_METHOD *SSL_get_current_compression(SSL *s);
^

Christoph