Merge #11611: [build] Don't fail when passed --disable-lcov and lcov isn't available

223a4aa [build] Don't fail when passed --disable-lcov and lcov isn't available (fanquake)

Pull request description:

  Fixes #10828
  As pointed out in #10828, failing with "lcov not found" when we've been passed --disable-lcov doesn't make sense. Master currently behaves like this (where lcov isn't available):
  ```
  ./configure --disable-lcov
  checking for pkg-config... /usr/local/bin/pkg-config
  checking pkg-config is at least version 0.9.0... yes
  configure: error: "lcov testing requested but lcov not found"
  ```

  cc @janstary

Tree-SHA512: 606fdbddae67e72fff175f2f34e2c9af4e6972d40d5e1ec5c5d8be5051a728e5b16c35cfd856da0c0ce81dcab9db154a4937b1a6ca1e0233b6e160f2f4362002

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],