multi-wildcard pattern rules of GNU Make

Using named variables, we can write more readable code (based on answer of Paljas):

letters:=a b c
numbers:=1 2 3 4

define GEN_RULE
$(letter).dat.$(number) : $(letter).rlt.$(number)
    ./rlt2dat $$< $$@
endef

$(foreach number,$(numbers), \
  $(foreach letter,$(letters), \
    $(eval $(GEN_RULE)) \
  ) \
)

We can generate SRC in a similar way. Note that using that method SRC will contain all the combinations. That may or may not be beneficial.

I'm afraid what you are trying to do is not possible the way you suggest to do it, since - as you already mention - (GNU) make only allows a single stem '%', see http://www.gnu.org/software/make/manual/make.html#Pattern-Rules:

A pattern rule looks like an ordinary rule, except that its target contains the character ‘%’ (exactly one of them).

Without it, creating such 'multi-dimensional' targets is cumbersome.

One way around this is by rebuilding the name of the dependency in the command (rather than in the dependency list):

SRC := a.dat.1 a.dat.2

all : $(SRC:%=%.dat2rlt)

%.dat2rlt :
    dat2rtl $(word 1,$(subst ., ,$*)).rlt.$(word 2,$(subst ., ,$*)) $*

Of course, however, this way you would lose the dependency, it will not rebuild once the rlt has been updated.

The only way I can see to address that is by generating the rules explicitly:

SRC := a.dat.1 a.dat.2

all : $(SRC)

define GEN_RULE
$1.dat.$2 : $1.rlt.$2
    dat2rtl $$< $$@
endef

$(foreach src,$(SRC),$(eval $(call GEN_RULE,$(word 1,$(subst ., ,$(src))),$(word 3,$(subst ., ,$(src))))))