build: remove lib/.gitignore

This isn't required - we already have the whole build/
directory in the top level .gitignore.

When we remove build/lib/.gitignore, it means
build/lib directory must be created somewhere
before we try to place a library there. Top-level
builds get this directory created automatically,
but building directly from a sub-directory's
Makefile means we need to explicitly create
build/lib.  So add a mkdir -p to the LIB_C
macro to do exactly that to cover these cases.

Also simplify 'make clean' at the top level Makefile.
A lot of work went in there to work around this
.gitignore file that's not needed now that we've
fixed the underlying problem by getting rid of
it.

Suggested-by: John Levon <john.levon@nutanix.com>
Signed-off-by: Jim Harris <james.r.harris@intel.com>
Change-Id: Ibb92ff84d8c2a9bbe3e193c84f15ef3866f07b1f
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/9169


Reviewed-by: Nick Connolly <nick.connolly@mayadata.io>
Reviewed-by: Aleksey Marchuk <alexeymar@mellanox.com>
Reviewed-by: Changpeng Liu <changpeng.liu@intel.com>
Reviewed-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>
Community-CI: Broadcom CI <spdk-ci.pdl@broadcom.com>
Community-CI: Mellanox Build Bot
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>