[lib] libdevmapper api check

[lib] libdevmapper api check

In libmultipath Makefile it is checked if the
symbol dm_task_no_flush is defined within the device mapper
library. On my system beside the library there exists a
symbolic link within the same directory pointing to this
library and containing libdevmapper.so in it's name:

lrwxrwxrwx 1 root root    20 Sep 22 22:15 /lib64/libdevmapper.so -> libdevmapper.so.1.02
-rwxr-xr-x 1 root root 89928 Feb 14  2008 /lib64/libdevmapper.so.1.02

In this case "grep -c" returns a 2 instead of 1 that is assumed
in the Makefile. I changed this check to not equal zero.