ccanlint: Allow path to gcov to be overriden

Currently ccanlint always assumes that the coverage tool can be
invoked under the command "gcov".

However, the coverage tool generally needs to be closely matched to
the compiler version.  So, the current behaviour won't work with
compilers other than gcc, like clang.  It won't even work for a gcc
version which isn't the standard system one matching gcov.

To address this, allow the command for the coverage tool to be
overridden on the ccanlint command line with a new --gcov option.  We
also allow it to be overridden for make check with a GCOV make
variable.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>