LLVM libc++ as included with Mac OS X 10.7 suffers from an issue where
the trailing character is discarded when the delimiter (LF) is not found
within the given buffer size (1024).  The returned length is also 1024
rather than 1023.

This issue results in truncated reads as observed with CMake 3.11.0 on
Mac OS X 10.7 and `cmake -E cmake_link_script link.txt`.

Solve this by replacing `istream::getline` by `std::getline` which does
not trigger the buffering issue.  There is one edge case that I decided
to leave up to the callers though: a file containing `\0` previously
resulted in line truncation, but is now included in the result.

Tested with Mac OS X 10.7 and 10.11:

    -DCMAKE_OSX_DEPLOYMENT_TARGET=10.7 -DCMAKE_CXX_FLAGS=-stdlib=libc++

and `./kwsysTestsCxx testSystemTools`.

Issue: cmake/cmake#15039