Make

Fix lazbuild-backend CI: propagate defines, GUI toggle, GTK2, dedup (… #393

Job	Run time
Resolve target list	5s
Native: Linux AArch64	42s
Native: Linux AArch64 (FORCE_SCALAR)	54s
QEMU: Linux PowerPC64 BE	6m 16s
Native: Linux x86_64 (FORCE_SCALAR)	38s
Native: Windows x86_64	1m 17s
Native: macOS x86_64 (Intel)	1m 17s
Native: macOS AArch64 (Apple Silicon)	1m 4s
VM: FreeBSD x86_64	2m 47s
QEMU: Linux ARMv7	6m 55s
VM: DragonFlyBSD x86_64	2m 26s
Native: Linux x86_64	26s
VM: Solaris x86_64	3m 51s
VM: NetBSD x86_64	0s
	28m 38s

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