astro/gpstk: Fix patches and CMake build
After the last commit to this port , that updated DISTFILES, patch
errors were reported on pkg-fallout:
patch: **** can't cd to /wrkdirs/usr/ports/astro/gpstk/work/dev: No such file or
=> Patch patch-ext__lib__FileDirProc__FileHunter.cpp failed to apply cleanly.
*** Error code 1
This change replaces a WRKSRC override with a CMAKE_SOURCE_PATH
variable to declare the source directory and regenerates patchfiles
against the default WRKSRC.
While I'm here: switch to USES=cmake:outsource to match the build
instructions upstream 
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