Allow Makefile to be modified by Makefile.local if present (#5997) #14987

Job	Run time
Core Basic (18.x)	6m 35s
Core Content (18.x)	3m 37s
Core Basic (20.x)	6m 44s
Core Content (20.x)	3m 46s
Core Basic - Plone 5 (18.x)	7m 2s
Core Blocks (18.x)	7m 46s
Core Blocks - Listing (18.x)	5m 0s
Core Blocks (20.x)	8m 4s
Core Blocks - Listing (20.x)	4m 42s
Core Control Panels (18.x)	4m 14s
Core Control Panels (20.x)	4m 13s
Core Volto Slate (18.x)	7m 58s
Core Volto Slate (20.x)	7m 30s
Coresandbox (18.x)	4m 59s
Project Generator (18.x)	7m 39s
Project Generator (20.x)	6m 51s
Multilingual (18.x)	3m 39s
Multilingual in Seamless Mode (18.x)	3m 43s
Seamless Mode (18.x)	7m 13s
Working Copy (18.x)	3m 38s
	1h 54m 53s

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