Get back to using pkg-config to check for OpenSSL

Commit 6300dfec7 removed the option to disable SSL support from the
configure script since it became a requirement, but it also removed the
use of pkg-config for finding the OpenSSL library and its dependencies.

This had the unfortunate consequence of breaking the correct detection
of library flags in many static linking scenarios. In some cases, for
example, OpenSSL might have been built with zlib, which requires `-lz`
to be passed to the linker when doing a static link of the irssi
executable. Thus, pkg-config becomes an invaluable tool in such
situations, since no guessing work is needed as the OpenSSL .pc file
provides all the necessary flags.

So, this commit re-inserts the PKG_CHECK_MODULES macro in the configure
script when looking for OpenSSL. The test using AC_CHECK_LIB remains,
but only as a last resort in case the one using pkg-config fails.

Also, because the macro AM_PATH_GLIB_2_0 contains an unconditional call
to PKG_PROG_PKG_CONFIG, the OpenSSL checks are moved so that they come
after the Glib ones in order to avoid doubly checking for the pkg-config
binary (PKG_CHECK_MODULES skips that check if it has been performed
before, but PKG_PROG_PKG_CONFIG does not).

configure.ac

@@ -243,11 +243,6 @@ if test "x$want_socks" = "xyes"; then
 	])
 fi
 
-dnl **
-dnl ** OpenSSL checks
-dnl **
-AC_CHECK_LIB([ssl], [SSL_library_init])
-
 dnl **
 dnl ** fe-text checks
 dnl **
@@ -289,7 +284,21 @@ if test -z "$GLIB_LIBS"; then
   AC_ERROR([GLIB is required to build irssi.])
 fi
 
-LIBS="$LIBS $GLIB_LIBS -lssl -lcrypto"
+LIBS="$LIBS $GLIB_LIBS"
+
+dnl **
+dnl ** OpenSSL checks
+dnl **
+PKG_CHECK_MODULES([OPENSSL], [openssl], [
+	CFLAGS="$CFLAGS $OPENSSL_CFLAGS"
+	LIBS="$LIBS $OPENSSL_LIBS"
+], [
+	AC_CHECK_LIB([ssl], [SSL_library_init], [
+		LIBS="$LIBS -lssl -lcrypto"
+	], [
+		AC_MSG_ERROR([The OpenSSL library was not found])
+	])
+])
 
 dnl **
 dnl ** curses checks