Fix the incorrect detection of ICONV_LIBRARY_PATH

This is introduced in rL308711.
Check for c library is incorrect here just because libc will be found always
and it does not mean that iconv is presented.

Thank to Andrew Krasny for narrowing down the root cause.

Reviewers: ecbeckmann
Reviewed By: ecbeckmann
Subscribers: mgorny, llvm-commits
Differential Revision: https://reviews.llvm.org/D38875

llvm-svn: 316064