Makefile: use $(MAKE) variable

For some systems (i.e. FreeBSD) the default 'make' is not compatible with the
GNU extensions used by QEMU makefiles.

Calling the GNU make (gmake) works, however the help displayed refers to the
host 'make' and copy/paste leads to lot of unobvious errors:

  $ gmake check-help
  [...]
   make check                Run all tests

  $ make check
  make: "Makefile" line 28: Missing dependency operator
  make: "Makefile" line 37: Need an operator
  make: "Makefile" line 41: warning: duplicate script for target "git-submodule-update" ignored
  make: "rules.mak" line 70: warning: duplicate script for target "%.o" ignored
  make: Unknown modifier ' '
  make: Unclosed substitution for eval modules (= missing)
  make: "tests/Makefile.include" line 24: Variable/Value missing from "export"
  make: "tests/" line 1: warning: Zero byte read from file, skipping rest of line.
  make: "tests/" line 1: Need an operator
  make: "Makefile" line 660: warning: duplicate script for target "ifneq" ignored
  make: "Makefile" line 78: warning: using previous script for "ifneq" defined here
  make: Fatal errors encountered -- cannot continue

Using the $(MAKE) variable, the help displayed is consistent with the 'make'
program used.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
