Document that GNU make is required

In theory the Autotools build system supports any "make" implementation,
but there are no regular contributors who test with BSD make, so the
inevitable result is that only GNU make actually works (fd.o #48277).

Apparently there's only one GNUism at the moment, which is fixable,
but that means repeating ourselves a bit more. If we instead document
that GNU make is required, we can simplify the Makefiles over time
by using extensions like $(patsubst), leading to a less error-prone
build system.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=48277
Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Reviewed-by: Chengwei Yang <chengwei.yang@intel.com>

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