Correct Build_settings.txt

doc/src/Build_settings.txt

@@ -212,11 +212,11 @@ following settings:
 
 [CMake variables]:
 
--D LAMMPS_JPEG=value      # yes or no
+-D WITH_JPEG=value      # yes or no
                           # default = yes if CMake finds JPEG files, else no
--D LAMMPS_PNG=value       # yes or no
+-D WITH_PNG=value       # yes or no
                           # default = yes if CMake finds PNG and ZLIB files, else no
--D LAMMPS_FFMPEG=value    # yes or no
+-D WITH_FFMPEG=value    # yes or no
                           # default = yes if CMake can find ffmpeg, else no :pre
 
 Usually these settings are all that is needed.  If CMake cannot find
@@ -267,7 +267,7 @@ gzip compression by several LAMMPS commands, including
 
 [CMake variables]:
 
--D LAMMPS_GZIP=value     # yes or no
+-D WITH_GZIP=value       # yes or no
                          # default is yes if CMake can find gzip, else no
 -D GZIP_EXECUTABLE=path  # path to gzip executable if CMake cannot find it :pre