Makefile.in: Use the variable MAKE instead of the make command

From https://www.gnu.org/software/make/manual/make.html#MAKE-Variable:
```
Recursive make commands should always use the variable MAKE, not the
explicit command name 'make', as shown here:

subsystem:
	cd subdir && $(MAKE)
```

This should avoid the following warning when running 'make releasecheck'
in some cases:
make[1]: warning: jobserver unavailable: using -j1. Add '+' to parent
make rule.