do not default to non-prefixed pkg-config when cross compiling

This can still be requested with PKG_CONFIG=/path/to/pkg-config.
Just do not use it as a default, and print a warning.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>

configure

@@ -1163,8 +1163,8 @@ fi
 # pkg-config probe
 
 if ! has $pkg_config; then
-  # likely not cross compiling, or hope for the best
-  pkg_config=pkg-config
+  echo warning: proceeding without "$pkg_config" >&2
+  pkg_config=/bin/false
 fi
 
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