BUILD: allow specifying platform patch_dir from cmd line

before:

patch_dir=$PLATFORM.patches

after

patch_dir can be specified as ./configure <....> patch_dir=/usr/local/site_ompi_v_x.y_patches/
if not specified - use default

config/opal_load_platform.m4

@@ -97,7 +97,7 @@ AC_DEFUN([OPAL_LOAD_PLATFORM], [
             AC_SUBST(OPAL_PARAM_FROM_PLATFORM, "no")
         fi
 
-        patch_dir="${with_platform}.patches"
+        patch_dir=${patch_dir:="${with_platform}.patches"}
         patch_done="${srcdir}/.platform_patches"
         patch_found=no