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5.4.2 Output During Parallel Execution
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When running several recipes in parallel the output from each recipe
appears as soon as it is generated, with the result that messages from
different recipes may be interspersed, sometimes even appearing on the
same line.  This can make reading the output very difficult.

   To avoid this you can use the '--output-sync' ('-O') option.  This
option instructs 'make' to save the output from the commands it invokes
and print it all once the commands are completed.  Additionally, if
there are multiple recursive 'make' invocations running in parallel,
they will communicate so that only one of them is generating output at a
time.

   If working directory printing is enabled (*note The
'--print-directory' Option: -w Option.), the enter/leave messages are
printed around each output grouping.  If you prefer not to see these
messages add the '--no-print-directory' option to 'MAKEFLAGS'.

   There are four levels of granularity when synchronizing output,
specified by giving an argument to the option (e.g., '-Oline' or
'--output-sync=recurse').

'none'
     This is the default: all output is sent directly as it is generated
     and no synchronization is performed.

'line'
     Output from each individual line of the recipe is grouped and
     printed as soon as that line is complete.  If a recipe consists of
     multiple lines, they may be interspersed with lines from other
     recipes.

'target'
     Output from the entire recipe for each target is grouped and
     printed once the target is complete.  This is the default if the
     '--output-sync' or '-O' option is given with no argument.

'recurse'
     Output from each recursive invocation of 'make' is grouped and
     printed once the recursive invocation is complete.

   Regardless of the mode chosen, the total build time will be the same.
The only difference is in how the output appears.

   The 'target' and 'recurse' modes both collect the output of the
entire recipe of a target and display it uninterrupted when the recipe
completes.  The difference between them is in how recipes that contain
recursive invocations of 'make' are treated (*note Recursive Use of
'make': Recursion.).  For all recipes which have no recursive lines, the
'target' and 'recurse' modes behave identically.

   If the 'recurse' mode is chosen, recipes that contain recursive
'make' invocations are treated the same as other targets: the output
from the recipe, including the output from the recursive 'make', is
saved and printed after the entire recipe is complete.  This ensures
output from all the targets built by a given recursive 'make' instance
are grouped together, which may make the output easier to understand.
However it also leads to long periods of time during the build where no
output is seen, followed by large bursts of output.  If you are not
watching the build as it proceeds, but instead viewing a log of the
build after the fact, this may be the best option for you.

   If you are watching the output, the long gaps of quiet during the
build can be frustrating.  The 'target' output synchronization mode
detects when 'make' is going to be invoked recursively, using the
standard methods, and it will not synchronize the output of those lines.
The recursive 'make' will perform the synchronization for its targets
and the output from each will be displayed immediately when it
completes.  Be aware that output from recursive lines of the recipe are
not synchronized (for example if the recursive line prints a message
before running 'make', that message will not be synchronized).

   The 'line' mode can be useful for front-ends that are watching the
output of 'make' to track when recipes are started and completed.

   Some programs invoked by 'make' may behave differently if they
determine they're writing output to a terminal versus a file (often
described as "interactive" vs.  "non-interactive" modes).  For example,
many programs that can display colorized output will not do so if they
determine they are not writing to a terminal.  If your makefile invokes
a program like this then using the output synchronization options will
cause the program to believe it's running in "non-interactive" mode even
though the output will ultimately go to the terminal.