FreeBSD/ports f458f81 (r518116)devel/mdb Makefile

Use clang 7 as compiler on 11.x and 12.x to fix build.

clang 8 and later error for duplicate definitions of enum values.  mdb
depends on having identical but duplicate definitions.  This was fixed
differently in 13 with a change to the kernel, but that change is
disruptive and cannot be easily backported.

Reported by:    pkg-fallout, Attila Nagy
Approved by:    bdrewery
MFH:            2019Q3
Differential Revision:  https://reviews.freebsd.org/D22486
DeltaFile
+10-1devel/mdb/Makefile
+10-11 files

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