meson: pass OpenSSL/ICU include dirs to extensions

Hi,

I was building an extension against a meson-built tree on macOS and the
build failed because a public server header includes openssl/ssl.h, but
the compiler couldn't find it. The extension includes libpq/auth.h,
which pulls in libpq-be.h, which includes openssl/ssl.h when OpenSSL
support is enabled. On systems where OpenSSL is installed in a
non-default prefix (Homebrew on macOS being the common case), that header
is not on the default search path.

With autoconf this works because you pass the include path to configure
via CPPFLAGS and it ends up in Makefile.global and pg_config --cppflags.
But meson keeps the include directory inside the ssl dependency object
and never writes it into var_cppflags, so extensions get nothing.

ICU has the same issue, and it's actually been known since the original
meson commit: ICU_CFLAGS sits in pgxs_empty with a comment saying it
"needs to be added, included by public server headers", so the
$(ICU_CFLAGS) reference in Makefile.global has always been empty.

The attached patch fixes both, mirroring how autoconf handles each.

For OpenSSL I extract the include directory from the ssl dependency and
append it to var_cppflags in src/include/meson.build. Since var_cppflags
feeds both VAL_CPPFLAGS and CPPFLAGS in the generated Makefile.global,
this fixes pg_config --cppflags and the PGXS build in one place, which
matches autoconf (where OpenSSL rides along in CPPFLAGS).

For ICU I populate ICU_CFLAGS from the icu dependency and drop it from
pgxs_empty. ICU gets a cmake fallback in get_variable because the meson
setup can find ICU via cmake; OpenSSL can't, so it doesn't get one.

One thing I noticed and left alone: autoconf's pg_config --cppflags also
includes ICU_CFLAGS (VAL_CPPFLAGS is built from STD_CPPFLAGS, which picks
up the override), while meson's won't. Extensions don't care because they
get ICU through $(ICU_CFLAGS) in Makefile.global, and I didn't want to
duplicate the flags in the PGXS path. Easy to add ICU to var_cppflags
too if you want full parity there.

I tested by building and installing PostgreSQL both ways (autoconf and
meson, with SSL enabled, on macOS with Homebrew) and compiling the same
extension against each installation. Both build now, and the compile
command contains both the OpenSSL and ICU include directories. Before
the patch, meson's pg_config --cppflags was empty for these.

Regards,
Aidar Imamov