Bump PORT_REVISION as pkg-plist has changed.
Fixed build on 10.1 but in the process found a bogus pkg-plist fixed.
If DOCS was defined, the plist was wrong.
Also cleaned up a dependency

Fixed using https://cmake.org/cmake/help/v3.0/command/link_directories.html
This shouldn't be necessary:

"Note that this command is rarely necessary. Library locations returned by find_package() and find_library() are absolute paths. Pass these absolute library file paths directly to the target_link_libraries() command. CMake will ensure the linker finds them."

The directory path appears to be in /usr/local/libdata/pkgconfig/gsl.pc
so it would appear the swig handling code neglects to respect this.
I'll dig into this sometime later.

PR: ports/210444
Submitted by: rozhuk.im@gmail.com