meson: build clover

meson: build clover

This has only been compile tested.

v2: - Have a single option for opencl (Eric E)
    - fix typo "tgis" -> "tgsi" (Curro)
    - Don't add "lib" to pipe loader libraries, which matches the
      autotools behavior
v3: - Remove trailing whitespace
    - Make PIPE_SEARCH_DIR an absolute path
v4: - add trailing / to LIBCLC defines

Acked-by: Curro Jerez <currojerez@riseup.net>
Tested-by: Jan Vesely <jan.vesely@rutgers.edu>
cc: Aaron Watry <awatry@gmail.com>
Signed-off-by: Dylan Baker <dylan.c.baker@intel.com>