Make a macro for "for"

#define F(i,n) for(int i = 0; i < n; ++i)

macros for start/process/stop of each line
 #define START int c0 = 0, ...
 #define P(x) if(x == 's'){ ...
 #define STOP mc0 = max(mc0, c0); ...

vertical/horizontal lines are simple:
  F(i,n){ START; F(j,n) P(s[i][j]); STOP; }
  F(i,n){ START; F(j,n) P(s[j][i]); STOP; }

The simplest diagonal is
  F(i,n)  { START; F(j,i+1) P(s[  i-j ][  j ]); STOP; }

with a macro for symmetry
#define I(ind) n-1-(ind)

the other lines simply invert some of coordinates:
  F(i,n)  { START; F(j,i+1) P(s[I(i-j)][  j ]); STOP; }
  F(i,n-1){ START; F(j,i+1) P(s[  i-j ][I(j)]); STOP; }
  F(i,n-1){ START; F(j,i+1) P(s[I(i-j)][I(j)]); STOP; }