configure: fix --meson=/path/to/meson

Due to a cut-and-paste error, the path to a user-specified meson
was ignored and replaced by whatever was in the path.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

configure

@@ -2015,7 +2015,7 @@ case "$meson" in
         fi
         meson="$python ${source_path}/meson/meson.py"
         ;;
-    *) meson=$(command -v meson) ;;
+    *) meson=$(command -v "$meson") ;;
 esac
 
 # Probe for ninja (used for compdb)