configure: Make missing pkg-config an error rather than a warning

If pkg-config doesn't exist then make configure fail immediately
with a useful error message. Now that glib is a required dependency,
proceeding despite the missing pkg-config will just cause us to
fail later with a misleading message about glib not being present.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>

configure

@@ -1340,8 +1340,8 @@ fi
 # pkg-config probe
 
 if ! has $pkg_config; then
-  echo warning: proceeding without "$pkg_config" >&2
-  pkg_config=/bin/false
+  echo "Error: pkg-config binary '$pkg_config' not found"
+  exit 1
 fi
 
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