reproc: Stop using PATH_MAX

PATH_MAX isn't always defined where it should be (see #75). Since
in the only scenario where we use it we just need some fixed size
to increment the buffer size with, let's just define our own constant
and remove the usage of PATH_MAX altogether.

reproc/src/process.posix.c

@@ -17,6 +17,8 @@
 #include "pipe.h"
 #include "strv.h"
 
+#define CWD_BUF_SIZE_INCREMENT 4096
+
 const pid_t PROCESS_INVALID = -1;
 
 static int signal_mask(int how, const sigset_t *newmask, sigset_t *oldmask)
@@ -51,7 +53,7 @@ static char *path_prepend_cwd(const char *path)
   ASSERT(path);
 
   size_t path_size = strlen(path);
-  size_t cwd_size = PATH_MAX;
+  size_t cwd_size = CWD_BUF_SIZE_INCREMENT;
 
   // We always allocate sufficient space for `path` but do not include this
   // space in `cwd_size` so we can be sure that when `getcwd` succeeds there is
@@ -70,7 +72,7 @@ static char *path_prepend_cwd(const char *path)
       return NULL;
     }
 
-    cwd_size += PATH_MAX;
+    cwd_size += CWD_BUF_SIZE_INCREMENT;
 
     char *result = realloc(cwd, cwd_size + path_size + 1);
     if (result == NULL) {