[build] Don't fail when passed --disable-lcov and lcov isn't available

Author: fanquake

configure.ac

@@ -162,7 +162,7 @@ AC_ARG_ENABLE([ccache],
 AC_ARG_ENABLE([lcov],
   [AS_HELP_STRING([--enable-lcov],
   [enable lcov testing (default is no)])],
-  [use_lcov=yes],
+  [use_lcov=$enableval],
   [use_lcov=no])
 
 AC_ARG_ENABLE([lcov-branch-coverage],