Fixes pkg-config usage and switches from AC_CHECK_LIB() to
AC_SEARCH_LIBS() for libcrypto.

This fixes the issue mentioned in pull request #176 of the crypto library
being added multiple times when using OpenSSL.

There are several details to note.

If the user specifies a OpenSSL directory with --with-openssl-dir, then
the relevant subdirectories need to end up in CFLAGS and LDFLAGS (as they
already did), not in CFLAGS and LIBS as proposed by PR #176. Since
AC_CHECK_LIB() prepends the library, putting the user supplied directory
in LIBS before the call to AC_CHECK_LIB() would put -lcrypto before
-L<dir>/lib, potentially picking up the wrong library or the system
supplied library.

If pkg-config is available and the user specifies a OpenSSL directory that
contains lib/pkgconfig/libcrypto.pc, then PKG_CONFIG_PATH is set so that
the custom location can be picked up by pkg-config. In this case, neither
CFLAGS nor LDFLAGS are modified since the pkg-config check will do the
right thing here.

The original code would call $PKG_CONFIG unconditionally via a shell
invocation regardless of whether pkg-config was available or not. This
changes it to use the PKG_CHECK_MODULES_STATIC() macro only if pkg-config
is available.

Note that PKG_CHECK_MODULES_STATIC() requires pkg-config 0.29 or higher,
hence the version argument to PKG_PROG_PKG_CONFIG().

Unlike PKG_CHECK_MODULES(), PKG_CHECK_MODULES_STATIC() includes
Libs.private for static linking (corresponds to 'pkg-config
--static'). For libcrypto, this will include libz and/or libdl as
necessary/configured.

For the case when pkg-config is not installed, the AC_CHECK_LIB() calls
for libz and libdl are kept. If found, they'll be added unconditionally,
as before.

As mentioned in the comment for PR #176, even if pkg-config is available
and used, it's still desirable to check for the presence of certain
functions in the configured library, and the usability of the library
itself (i.e. it's built for the correct platform).

To do so, and also to check for libcrypto if pkg-config isn't installed,
AC_SEARCH_LIBS() is used. This will check for the given function in the
libraries listed, which is only libcrypto in this case.

The way AC_SEARCH_LIBS() works is that it will try to find the function
without adding any libraries and then start adding libraries from the
given list until the function is found or the list exhausted.

For the case when pkg-config is installed and libcrypto is found via
PKG_CHECK_MODULES_STATIC(), these extra checks for specific functions
will not add -lcrypto again because it doesn't need to - it's already in
LIBS and the functions will be found.

For the case when pkg-config is not installed, the first check will
add -lcrypto to LIBS while subsequent checks will pass without adding
it again.

Should any of the checks fail, i.e. the function isn't found or the
configured library isn't usable, then it will fail as intended.