Functions in Makefile

There are 3 related concepts:

1. call function
2. multi-line variables
3. conditionals

The refactored result could look like this:

ifeq ($(UNAME),Linux)
    compile = $(MAKE) FC=$(1) FFLAGS=$(2) PETSC_FFLAGS=$(3) \
                      TARGET=$@ LEXT="$(1)_$(UNAME)" -e syst
else
    define compile =
        echo $(err_arch)
        exit 1
    endef
endif
        

debug_ifort:
        $(call compile,ifort,$(difort),"...")

That one \ that is left is to continue the $(MAKE) line for the shell.
No multi-line variable is necessary here, because it is just one line of shell code.
Multi-line variables are only used in the else block.

If you don’t need parameters you can use := assignment and just expand the method with $(compile) (see canned recipes)

Note: Using make prior to version 3.82, the = was not recognized at the end of the define statement for me. I fixed this by using define compile instead.