append cflags,cxxflags,shared in makefile (libretro core with msys2)

[Xephobia]

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Expected behavior

using make -j4 CFLAGS+='-pipe -O3 -march=native -mtune=native -flto' CXXLAGS+='-pipe -O3 -march=native -mtune=native -flto' SHARED+='-pipe -O3 -march=native -mtune=native -flto' should build fine

Actual behavior

Strange error surely related as overriding the variables

Steps to reproduce the behavior

compile with the command specified above

[negativeExponent]

use:

CFLAGS+='-pipe -O3 -march=native -mtune=native -flto' CXXLAGS+='-pipe -O3 -march=native -mtune=native -flto' SHARED+='-pipe -O3 -march=native -mtune=native -flto' make -j4