Fix compiler warnings.

[moritzbuhl]

cc -O2 -pipe  -DVERSION='"2.8.11"'    -c -o detex.o detex.c
detex.l:140:7: warning: a function declaration without a prototype is deprecated in all versions of C and is treated as a zero-parameter prototype in C2x, conflicting with a previous declaration [-Wdeprecated-non-prototype]
char *getenv();
      ^
/usr/include/stdlib.h:110:7: note: conflicting prototype is here
char    *getenv(const char *);
         ^
detex.l:413:36: warning: expression which evaluates to zero treated as a null pointer constant of type 'char *' [-Wnon-literal-null-conversion]
                                                        rgsbIncList[csbIncList++] = '\0';
                                                                                    ^~~~
detex.l:360:54: warning: result of comparison of constant -1 with expression of type 'char' is always true [-Wtautological-constant-out-of-range-compare]
                while ((c = input()) != verbchar && c != '\n' && c != EOF && c != 0) {
                                                                 ~ ^  ~~~
detex.l:825:8: warning: a function declaration without a prototype is deprecated in all versions of C and is treated as a zero-parameter prototype in C2x, conflicting with a previous declaration [-Wdeprecated-non-prototype]
        FILE    *TexOpen();
                 ^
detex.l:137:7: note: conflicting prototype is here
FILE *TexOpen(char *sbFile);
      ^
detex.l:851:8: warning: a function declaration without a prototype is deprecated in all versions of C and is treated as a zero-parameter prototype in C2x, conflicting with a previous declaration [-Wdeprecated-non-prototype]
        FILE    *TexOpen();
                 ^
detex.l:137:7: note: conflicting prototype is here
FILE *TexOpen(char *sbFile);
      ^
5 warnings generated.