apr: fix LTFLAGS to make it work with ccache

apr: fix LTFLAGS to make it work with ccache

When ccache is enabled, libtool requires --tag=CC when use ccache,
otherwise when building apr-util with ccache enabled:

| libtool: compile: unable to infer tagged configuration
| libtool: error: specify a tag with '--tag'
| libtool: compile: unable to infer tagged configuration
| make[1]: *** [buckets/apr_buckets.lo] Error 1

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>