lint: Also lint files with shellcheck directive

Files like config.site.in are not referenced by any other script in our
tree, so we need to mark it manually with a "shellcheck shell="
directive and make sure that shellcheck is run on them.

depends/config.site.in

@@ -1,3 +1,13 @@
+# shellcheck shell=sh disable=SC2034 # Many variables set will be used in
+                                     # ./configure but shellcheck doesn't know
+                                     # that, hence: disable=SC2034
+
+true  # Dummy command because shellcheck treats all directives before first
+      # command as file-wide, and we only want to disable for one line.
+      #
+      # See: https://github.com/koalaman/shellcheck/wiki/Directive
+
+# shellcheck disable=SC2154
 depends_prefix="$(cd "$(dirname ${ac_site_file})/.." && pwd)"
 
 cross_compiling=maybe
@@ -50,7 +60,7 @@ if test x@host_os@ = xdarwin; then
 fi
 
 PATH=$depends_prefix/native/bin:$PATH
-PKG_CONFIG="`which pkg-config` --static"
+PKG_CONFIG="$(which pkg-config) --static"
 
 # These two need to remain exported because pkg-config does not see them
 # otherwise. That means they must be unexported at the end of configure.ac to