genann: Optionally resolve activation functions at link time

Shave around 94 million instructions and 10 million branches off of execution
trace of example4 if the sigmoid activation function is resolved at link-time.

Before (`make`):
```
 Performance counter stats for './example4':

         98.988806      task-clock (msec)         #    0.998 CPUs utilized
                 1      context-switches          #    0.010 K/sec
                 0      cpu-migrations            #    0.000 K/sec
                79      page-faults               #    0.798 K/sec
       312,298,260      cycles                    #    3.155 GHz
     1,094,183,752      instructions              #    3.50  insn per cycle
       212,007,732      branches                  # 2141.734 M/sec
            62,774      branch-misses             #    0.03% of all branches

       0.099228100 seconds time elapsed
```

After:

`make`:
```
 Performance counter stats for './example4':

         97.335180      task-clock (msec)         #    0.998 CPUs utilized
                 0      context-switches          #    0.000 K/sec
                 0      cpu-migrations            #    0.000 K/sec
                82      page-faults               #    0.842 K/sec
       306,722,357      cycles                    #    3.151 GHz
     1,065,669,644      instructions              #    3.47  insn per cycle
       214,256,601      branches                  # 2201.225 M/sec
            60,154      branch-misses             #    0.03% of all branches

       0.097577079 seconds time elapsed
```

`make sigmoid`:
```
 Performance counter stats for './example4':

         92.629610      task-clock (msec)         #    0.997 CPUs utilized
                 0      context-switches          #    0.000 K/sec
                 0      cpu-migrations            #    0.000 K/sec
                78      page-faults               #    0.842 K/sec
       291,863,801      cycles                    #    3.151 GHz
     1,000,931,204      instructions              #    3.43  insn per cycle
       202,465,800      branches                  # 2185.757 M/sec
            50,949      branch-misses             #    0.03% of all branches

       0.092889789 seconds time elapsed
```

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>

test.c

@@ -248,7 +248,7 @@ void sigmoid() {
     const double d = .0001;
 
     while (i < max) {
-        lfequal(genann_act_sigmoid(i), genann_act_sigmoid_cached(i));
+        lfequal(genann_act_sigmoid(NULL, i), genann_act_sigmoid_cached(NULL, i));
         i += d;
     }
 }