On line 30 you already use local variables. They reallY don't hurt:)
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void mine_set()
{
do
{
// Note: later (with more understanding) you should introduce constants for this!
int r=rand()%8;
int c=rand()%8;
if (minesweeper[r][c] !=mine)
{
minesweeper[r][c]=mine;
mines++; // Note: ++
}
}
while (mines<10);
}