Cmake error when compiled with BUILD_SHARED_LIB=ON

[yanjen]

Dear authors,

I ran into this CMake error while compiling with MPI and with BUILD_SHARED_LIB=ON.

-- MPI enabled
-- OpenMP enabled
-- Using submodule /path/to/pFUnit/extern/fArgParse
-- Using submodule /path/to/pFUnit/extern/fArgParse/extern/gFTL-shared
-- Performing Test _LOGICAL_DEFAULT_KIND: SUCCESS (value=4)
-- Performing Test _INT_DEFAULT_KIND: SUCCESS (value=4)
-- Performing Test _ISO_INT8: SUCCESS (value=1)
-- Performing Test _ISO_INT16: SUCCESS (value=2)
-- Performing Test _ISO_INT32: SUCCESS (value=4)
-- Performing Test _ISO_INT64: SUCCESS (value=8)
-- Performing Test _REAL_DEFAULT_KIND: SUCCESS (value=4)
-- Performing Test _DOUBLE_DEFAULT_KIND: SUCCESS (value=8)
-- Performing Test _ISO_REAL32: SUCCESS (value=4)
-- Performing Test _ISO_REAL64: SUCCESS (value=8)
-- Performing Test _ISO_REAL128: SUCCESS (value=16)
-- Using submodule /path/to/pFUnit/extern/fArgParse/extern/gFTL-shared/extern/gFTL
-- cpp is /usr/bin/cpp
-- Configuring done
CMake Error: install(EXPORT "PFUNIT" ...) includes target "pfunit-mpi-defines" more than once in the export set.
CMake Error: install(EXPORT "PFUNIT" ...) includes target "pfunit-mpi-defines" more than once in the export set.
CMake Error in CMakeLists.txt:
  given target "pfunit-mpi-defines" more than once.

CMake Generate step failed.  Build files cannot be regenerated correctly.

This error was caused by installing pfunit-mpi-defines in the foreach loop repeatedly. This branch fixes the error. Please consider merging this fix.