gmp: Fix build CC detection
gmp: Fix build CC detection

Signed-off-by: Esben Haabendal <esben.haabendal@prevas.dk>

(cherry picked from commit 638e054be55e257b1e543054a52687a83236d174)

jacobbarsoe/rpi-base,oe-lite-rpi/base,jacobbarsoe/rpi-base,oe-lite-rpi/base