Add warning if using make instead of gmake

[james-d-mitchell]

In the main GAP directory, there's the following:

https://github.com/gap-system/gap/blob/master/Makefile

which prints:

Please use GNU make to build GAP (try 'gmake' or 'gnumake')
*** Error 1 in /home/jdm/gap (Makefile:22 '.DEFAULT': @exit 1)

if one uses make instead of gmake. Whereas for Semigroups we get the less helpful:

*** Parse error in /home/jdm/gap/pkg/Semigroups: Need an operator in 'HPCOMBI_CONSTEXPR_FUN_ARGS' (Makefile:36)
*** Parse error: Need an operator in 'endif' (Makefile:38)
*** Parse error: Need an operator in 'LIBSEMIGROUPS_HPCOMBI_ENABLED' (Makefile:40)
*** Parse error: Need an operator in 'endif' (Makefile:42)
*** Parse error: Need an operator in 'WITH_INCLUDED_LIBSEMIGROUPS' (Makefile:44)
*** Parse error: Need an operator in 'LIBSEMIGROUPS_HPCOMBI_ENABLED' (Makefile:48)
*** Parse error: Need an operator in 'endif' (Makefile:51)
*** Parse error: Need an operator in 'endif' (Makefile:54)
*** Parse error: Need an operator in 'KERNEL_DEBUG' (Makefile:57)
*** Parse error: Need an operator in 'else' (Makefile:59)
*** Parse error: Need an operator in 'endif' (Makefile:61)
*** Parse error: Need an operator in 'SYS_IS_CYGWIN' (Makefile:72)
*** Parse error: Need an operator in 'endif' (Makefile:74)
*** Parse error: Need an operator in 'GAP_KERNEL_MAJOR_VERSION' (Makefile.gappkg:42)
*** Parse error: Need an operator in 'endif' (Makefile.gappkg:45)
*** Parse error: Need an operator in 'KEXT_USE_AUTOCONF' (Makefile.gappkg:67)
*** Parse error: Need an operator in 'else' (Makefile.gappkg:69)
*** Parse error: Need an operator in 'endif' (Makefile.gappkg:71)
*** Parse error: Missing dependency operator (Makefile.gappkg:93)
*** Parse error: Need an operator in 'V' (Makefile.gappkg:94)
*** Parse error: Need an operator in 'endif' (Makefile.gappkg:96)
*** Parse error: Need an operator in 'endif' (Makefile.gappkg:97)
*** Parse error: Filename missing from "-include" (Makefile.gappkg:108)
*** Parse error: Need an operator in 'KEXT_USE_AUTOCONF' (Makefile.gappkg:170)
*** Parse error: Missing dependency operator (Makefile.gappkg:175)
*** Parse error: Missing dependency operator (Makefile.gappkg:183)
*** Parse error: Missing dependency operator (Makefile.gappkg:202)
*** Parse error: Missing dependency operator (Makefile.gappkg:213)
*** Parse error: Missing dependency operator (Makefile.gappkg:215)
*** Parse error: Need an operator in 'WITH_INCLUDED_LIBSEMIGROUPS' (Makefile:83)
*** Parse error: Need an operator in 'endif' (Makefile:96)

It'd be great if we could have a nice warning instead of the less informative above.