Fix a few strict compiler warnings.

src/coloredstderr.c

@@ -119,14 +119,14 @@ static void close_fd(int fd) {
 
 /* "Action" handlers called when a file descriptor is matched. */
 
-static char *pre_string;
+static char const *pre_string;
 static size_t pre_string_size;
-static char *post_string;
+static char const *post_string;
 static size_t post_string_size;
 
 /* Load alternative pre/post strings from the environment if available, fall
  * back to default values. */
-inline static void init_pre_post_string() {
+inline static void init_pre_post_string(void) {
     pre_string = getenv(ENV_NAME_PRE_STRING);
     if (!pre_string) {
         pre_string = DEFAULT_PRE_STRING;
@@ -476,7 +476,7 @@ int execve(char const *filename, char * const argv[], char * const env[]) {
     /* Count arguments. */ \
     size_t count = 1; /* arg */ \
     va_start(ap, arg); \
-    while (va_arg(ap, const char *)) { \
+    while (va_arg(ap, char const *)) { \
         count++; \
     } \
     va_end(ap); \

src/debug.h

@@ -62,7 +62,7 @@ static void debug(char const *format, ...) {
 static void warning(char const *format, ...) {
     va_list ap;
 
-    char *home = getenv("HOME");
+    char const *home = getenv("HOME");
     if (!home) {
         return;
     }

tests/example.c

@@ -24,6 +24,8 @@
 
 
 int main(int argc, char **argv) {
+    (void)argv;
+
     fprintf(stderr, "write to stderr: %d\n", argc);
     printf("write to stdout\n");
     fflush(stdout);

tests/example_vfork.c

@@ -19,6 +19,7 @@
 
 #include <stdio.h>
 #include <stdlib.h>
+#include <sys/types.h>
 #include <unistd.h>
 
 int main(int argc, char **argv) {