more instructions on external lib downloads, and CMake versions

doc/src/Build_cmake.txt

@@ -79,8 +79,9 @@ directory to un-install all packages.  The purge removes all the *.h
 files auto-generated by make.
 
 You must have CMake version 2.8 or later on your system to build
-LAMMPS.  If you include the GPU or KOKKOS packages, CMake version 3.2
-or later is required.  Installation instructions for CMake are below.
+LAMMPS.  A handful of LAMMPS packages (KOKKOS, LATTE, MSCG) require a
+later version.  CMake will print a message telling you if a later
+version is required.  Installation instructions for CMake are below.
 
 After the initial build, if you edit LAMMPS source files, or add your
 own new files to the source directory, you can just re-type make from

doc/src/Build_extras.txt

@@ -175,8 +175,15 @@ package?" page.
 [CMake build]:
 
 -D DOWNLOAD_KIM=value    # download OpenKIM API v1 for build, value = no (default) or yes
--D KIM_LIBRARY=path      # path to KIM shared library (only needed if a custom location) 
--D KIM_INCLUDE_DIR=path  # path to KIM include directory (only needed if a custom location) :pre
+-D KIM_LIBRARY=path      # KIM library file (only needed if a custom location) 
+-D KIM_INCLUDE_DIR=path  # KIM include directory (only needed if a custom location) :pre
+
+If DOWNLOAD_KIM is set, the KIM library will be downloaded and built
+inside the CMake build directory.  If the KIM library is already on
+your system (in a location CMake cannot find it), KIM_LIBRARY is the
+filename (plus path) of the KIM library file, not the directory the
+library file is in.  KIM_INCLUDE_DIR is the directory the KIM include
+file is in.
 
 [Traditional make]:
 
@@ -296,7 +303,13 @@ library.
 [CMake build]:
 
 -D DOWNLOAD_LATTE=value    # download LATTE for build, value = no (default) or yes
--D LATTE_LIBRARY=path      # path to LATTE shared library (only needed if a custom location) :pre
+-D LATTE_LIBRARY=path      # LATTE library file (only needed if a custom location) :pre
+
+If DOWNLOAD_LATTE is set, the LATTE library will be downloaded and
+built inside the CMake build directory.  If the LATTE library is
+already on your system (in a location CMake cannot find it),
+LATTE_LIBRARY is the filename (plus path) of the LATTE library file,
+not the directory the library file is in.
 
 [Traditional make]:
 
@@ -371,8 +384,15 @@ lib/mscg/README and MSCG/Install files for more details.
 [CMake build]:
 
 -D DOWNLOAD_MSCG=value    # download MSCG for build, value = no (default) or yes
--D MSCG_LIBRARY=path      # path to MSCG shared library (only needed if a custom location) 
--D MSCG_INCLUDE_DIR=path  # path to MSCG include directory (only needed if a custom location) :pre
+-D MSCG_LIBRARY=path      # MSCG library file (only needed if a custom location) 
+-D MSCG_INCLUDE_DIR=path  # MSCG include directory (only needed if a custom location) :pre
+
+If DOWNLOAD_MSCG is set, the MSCG library will be downloaded and built
+inside the CMake build directory.  If the MSCG library is already on
+your system (in a location CMake cannot find it), MSCG_LIBRARY is the
+filename (plus path) of the MSCG library file, not the directory the
+library file is in.  MSCG_INCLUDE_DIR is the directory the MSCG
+include file is in.
 
 [Traditional make]:
 
@@ -515,8 +535,15 @@ library"_voro_home.
 [CMake build]:
 
 -D DOWNLOAD_VORO=value    # download Voro++ for build, value = no (default) or yes
--D VORO_LIBRARY=path      # (only needed if at custom location) path to VORO shared library
--D VORO_INCLUDE_DIR=path  # (only needed if at custom location) path to VORO include directory :pre
+-D VORO_LIBRARY=path      # Voro++ library file (only needed if at custom location) 
+-D VORO_INCLUDE_DIR=path  # Voro++ include directory (only needed if at custom location) :pre
+
+If DOWNLOAD_VORO is set, the Voro++ library will be downloaded and
+built inside the CMake build directory.  If the Voro++ library is
+already on your system (in a location CMake cannot find it),
+VORO_LIBRARY is the filename (plus path) of the Voro++ library file,
+not the directory the library file is in.  VORO_INCLUDE_DIR is the
+directory the Voro++ include file is in.
 
 [Traditional make]:
 
@@ -877,7 +904,10 @@ Eigen3 is a template library, so you do not need to build it.
 -D DOWNLOAD_EIGEN3            # download Eigen3, value = no (default) or yes
 -D EIGEN3_INCLUDE_DIR=path    # path to Eigen library (only needed if a custom location) :pre
 
-Set EIGEN3_INCLUDE_DIR if CMake cannot find the Eigen3 library.
+If DOWNLOAD_EIGEN3 is set, the Eigen3 library will be downloaded and
+inside the CMake build directory.  If the Eig3n3 library is already on
+your system (in a location CMake cannot find it), EIGEN3_INCLUDE_DIR
+is the directory the Eigen3++ include file is in.
 
 [Traditional make]: