[mod_cgi] fall back to pipe() if pipe2() fails

This change should fix an issue with lighttpd on Debian kfreebsd-* arch
  (kfreebsd-amd64 and kfreebsd-i386)
.libs/mod_cgi.o: In function `cgi_create_env':
./src/mod_cgi.c:1103: warning: pipe2 is not implemented and will always fail

lighttpd is single-threaded so there is no race with pipe()
and then fcntl() F_SETFD FD_CLOEXEC on the pair of pipe fds.
Using pipe2() where available is still slightly more efficient
by eliding the syscalls to set FD_CLOEXEC.

src/mod_cgi.c

@@ -35,18 +35,18 @@
 #include <stdio.h>
 #include <fcntl.h>
 
-#ifdef HAVE_PIPE2
-#define pipe_cloexec(pipefd) pipe2((pipefd), O_CLOEXEC)
-#elif defined FD_CLOEXEC
-#define pipe_cloexec(pipefd) \
-  (   0 == pipe(pipefd) \
-   && 0 == fcntl(pipefd[0], F_SETFD, FD_CLOEXEC) \
-   && 0 == fcntl(pipefd[1], F_SETFD, FD_CLOEXEC) \
-    ? 0 \
-    : -1)
-#else
-#define pipe_cloexec(pipefd) pipe(pipefd)
-#endif
+static int pipe_cloexec(int pipefd[2]) {
+  #ifdef HAVE_PIPE2
+    if (0 == pipe2(pipefd, O_CLOEXEC)) return 0;
+  #endif
+    return 0 == pipe(pipefd)
+       #ifdef FD_CLOEXEC
+        && 0 == fcntl(pipefd[0], F_SETFD, FD_CLOEXEC)
+        && 0 == fcntl(pipefd[1], F_SETFD, FD_CLOEXEC)
+       #endif
+      ?  0
+      : -1;
+}
 
 enum {EOL_UNSET, EOL_N, EOL_RN};