Update -Wno-long-double test to support clang.

This prevents bazillions of warnings from clang tha -Wno-long-double
isn't supported.

The warning that clang issues when it see -Wno-long-double is does not
use any of the words we were looking for, so I added "unknown" to the
list of words to look for.  I also re-indented the two m4 tests so
that they're a bit more readable.

cmr=v1.7.4:reviewer=brbarret:subject=Update -Wno-long-double test to support clang
Fix compiler warning.

cmr=v1.7.4:reviewer=hjelmn
Ensure is_tar is always initialized.

cmr=v1.7.4:reviewer=rhc
Fix compiler warning: event is unsigned; it can't be negative

cmr=v1.7.4:reviewer=rhc
Fix compiler warnings about signed/unsigned comparisons

Change static opal_setlimit() function to return its value in an OUT
parameter and return the usual int error code indicating success or
failure.  

The OUT param and return code need to be separated because the OUT
param is an unsigned type, but opal_setlimit() was returning -1 upon
failure.  Hence, the caller could not know that it had failed because
the return type was previously an unsigned type.

cmr=v1.7.4:reviewer=rhc:subject=Fix opal sys_limits.c signed/unsigned warnings
Fix a few compiler warnings reported by clang:

 * Ensure "cnt" is always initialized
 * Ensure we dont' buffer overflow on strncat() -- need to ensure we
   account for the terminating \0 character
 * hwloc_get_type_depth() returns an int (not unsigned), and
   HWLOC_TYPE_DEPTH_UNKNOWN if it's unknown (which is probably <0, but
   still, might as well check what the official hwloc docs say to
   check for)

cmr=v1.7.4:reviewer=rhc:subject=fix hwloc base compiler warnings