[RFC] Enable usage of externally supplied 'TONES DATA'

Motivation: Currently, ft8_lib cannot encode free-text messages.

Usage:

```
$ ft8code "U09TUNQUC9ZH"
...
3140652 00545736465273610247324500636 3140652 55505324500472027752070701676 3140652
```

```
$ export DEBUG_TONES_DATA="3140652 00545736465273610247324500636 3140652 55505324500472027752070701676 3140652"

$ ./gen_ft8 "U09TUNQUC9ZH" compatible.wav
Note: DEBUG_TONES_DATA is set! Using it to generate the WAVE file...
FSK tones: 3140652005457364652736102473245006363140652555053245004720277520707016763140652
```

WSJT-X then decodes the generated compatible.wav file just fine.

demo/decode_ft8.c

@@ -1,3 +1,5 @@
+#define _POSIX_C_SOURCE 199309L
+
 #include <stdlib.h>
 #include <string.h>
 #include <stdio.h>

demo/gen_ft8.c

@@ -120,6 +120,7 @@ int main(int argc, char** argv)
         return -1;
     }
 
+    uint8_t tones[128]; // Array of 79 tones (symbols)
     const char* message = argv[1];
     const char* wav_path = argv[2];
     float frequency = 1000.0;
@@ -128,6 +129,15 @@ int main(int argc, char** argv)
         frequency = atof(argv[3]);
     }
     bool is_ft4 = (argc > 4) && (0 == strcmp(argv[4], "-ft4"));
+    int num_tones = (is_ft4) ? FT4_NN : FT8_NN;
+    float symbol_period = (is_ft4) ? FT4_SYMBOL_PERIOD : FT8_SYMBOL_PERIOD;
+    float symbol_bt = (is_ft4) ? FT4_SYMBOL_BT : FT8_SYMBOL_BT;
+    float slot_time = (is_ft4) ? FT4_SLOT_TIME : FT8_SLOT_TIME;
+
+    if(getenv("DEBUG_TONES_DATA")) {
+        printf("Note: DEBUG_TONES_DATA is set! Using it to generate the WAVE file...\n");
+        goto DEBUG_TONES_DATA;
+    }
 
     // First, pack the text data into binary message
     ftx_message_t msg;
@@ -146,13 +156,7 @@ int main(int argc, char** argv)
     }
     printf("\n");
 
-    int num_tones = (is_ft4) ? FT4_NN : FT8_NN;
-    float symbol_period = (is_ft4) ? FT4_SYMBOL_PERIOD : FT8_SYMBOL_PERIOD;
-    float symbol_bt = (is_ft4) ? FT4_SYMBOL_BT : FT8_SYMBOL_BT;
-    float slot_time = (is_ft4) ? FT4_SLOT_TIME : FT8_SLOT_TIME;
-
     // Second, encode the binary message as a sequence of FSK tones
-    uint8_t tones[num_tones]; // Array of 79 tones (symbols)
     if (is_ft4)
     {
         ft4_encode(msg.payload, tones);
@@ -162,6 +166,18 @@ int main(int argc, char** argv)
         ft8_encode(msg.payload, tones);
     }
 
+DEBUG_TONES_DATA:
+    if(getenv("DEBUG_TONES_DATA")) {
+        char *tmp = getenv("DEBUG_TONES_DATA");
+        int j = 0;
+
+        for (int i = 0; i < strlen(tmp); i++) {
+            if (tmp[i] == ' ')
+                continue;
+            tones[j++] = tmp[i] - '0';
+        }
+    }
+
     printf("FSK tones: ");
     for (int j = 0; j < num_tones; ++j)
     {