List mpi-serial's include directory before any system-level includes

Without this, I was getting runtime failures on my new mac, using
gcc/gfortran 8.2.0. The problem was that it seemed to be picking up
/usr/local/include/mpif.h rather than the mpif.h from mpi-serial.

I'm not sure why this problem suddenly appeared on my new machine: both
my new and old machines had -I/usr/local/include before the mpi-serial
include directory, and both had an mpif.h in /usr/local/include. So I'm
not sure why this issue didn't appear on my old machine, too.

CMakeLists.txt

@@ -70,8 +70,11 @@ include_directories(${CMAKE_CURRENT_BINARY_DIR}/mct/mpeu)
 link_directories(${CMAKE_CURRENT_BINARY_DIR}/mct/mct)
 link_directories(${CMAKE_CURRENT_BINARY_DIR}/mct/mpeu)
 if (USE_MPI_SERIAL)
-  include_directories(${CMAKE_CURRENT_BINARY_DIR}/mct/mpi-serial)
-  link_directories(${CMAKE_CURRENT_BINARY_DIR}/mct/mpi-serial)
+  # We need to list the mpi-serial include directory before system-level
+  # directories so that we're sure to use mpi-serial's mpif.h instead of
+  # an mpif.h from a system path.
+  include_directories(BEFORE ${CMAKE_CURRENT_BINARY_DIR}/mct/mpi-serial)
+  link_directories(BEFORE ${CMAKE_CURRENT_BINARY_DIR}/mct/mpi-serial)
 endif()
 
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