Improvements to CMake documentation / output

• I think that two sentences in INSTALL.CMake could be misleading:

  If MPI is found on the system, it will be used by default (WITH_MPI=ON).
  
  If NetCDF is found on the system, it will be used by default (WITH_NETCDF=ON), and
  so will the NCDF library (See below for a fuller option list).

  but later in the same file:

    - WITH_MPI:   It is ON by default. To deactivate, use -DWITH_MPI=OFF
  [...]
    - WITH_NETCDF: It is ON by default.
                   At the same time, it will set WITH_NCDF to ON.
                   WITH_NCDF_PARALLEL will remain OFF by default.

  where the defaults do not depend on the libraries found on the system. A reader could interpret the first two sentences as saying that those defaults take those values only if the corresponding libraries are found on the system. I believe that those defaults indeed do not depend on what is found on the system, at least with the MPI one the default is to fail if MPI is not found and it has not been explicitly disabled:

  $ cmake -S. -B_build
  [...]
  -- Could NOT find MPI_C (missing: MPI_C_LIB_NAMES MPI_C_HEADER_DIR MPI_C_WORKS) 
  -- Could NOT find MPI_Fortran (missing: MPI_Fortran_LIB_NAMES MPI_Fortran_F77_HEADER_DIR MPI_Fortran_MODULE_DIR MPI_Fortran_WORKS) 
  -- Could NOT find MPI (missing: MPI_C_FOUND MPI_Fortran_FOUND) 
      Reason given by package: MPI component 'CXX' was requested, but language CXX is not enabled.  
  
  -- MPI could not be found by CMake
  -- Try setting MPI_Fortran_COMPILER to an MPI wrapper, or the MPI_HOME env variable
  -- If unsuccessful, set -DWITH_MPI=OFF
  CMake Error at CMakeLists.txt:73 (message):
    MPI could not be found by CMake
  
  
  -- Configuring incomplete, errors occurred!

  Please let's clarify whether we want to improve those texts (and keep the current implementation of defaults) or make the defaults dependent on the libraries found on the system.
• If NetCDF is enabled but not present in the system, the error message that is printed in red (lines 5-18 below) contains wrong information:

  Package netcdf was not found in the pkg-config search path.
  Perhaps you should add the directory containing `netcdf.pc'
  to the PKG_CONFIG_PATH environment variable
  No package 'netcdf' found
  CMake Error at Config/cmake/FindNetCDF.cmake:225 (message):
    Unable to properly find NetCDF.  Found static libraries at:
    /users/escartin/siesta/NetCDF_C_LIBRARY-NOTFOUND but could not run
    nc-config:
  Call Stack (most recent call first):
    CMakeLists.txt:128 (find_package)
  
  
  CMake Error at Config/cmake/FindNetCDF.cmake:225 (message):
    Unable to properly find NetCDF.  Found static libraries at:
    /users/escartin/siesta/NetCDF_Fortran_LIBRARY-NOTFOUND but could not run
    nc-config:
  Call Stack (most recent call first):
    CMakeLists.txt:128 (find_package)
  
  
  -- Could NOT find NetCDF (missing: NetCDF_INCLUDE_DIRS NetCDF_LIBRARIES) (Required is at least version "4.0.0")
  -- Looking for Fortran sgemm
  [...]

  Would it be possible to print a clean error about not finding NetCDF, without entering Config/cmake/FindNetCDF.cmake:225 ?