Fix build with GCC on FreeBSD

Include <limits.h> for visibility of "PATH_MAX".

Add FreeBSD to known platforms in "gen.py".

Change-Id: Ied0608962b9d044fe4d2f08c4dbd0dea5158ba26
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/5181
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>

build/gen.py

diff --git a/build/gen.py b/build/gen.py
index 22b6a64..a1e5201 100755
--- a/build/gen.py
+++ b/build/gen.py
@@ -46,7 +46,7 @@
 
   @staticmethod
   def known_platforms():
-    return ['linux', 'darwin', 'msvc', 'aix', 'fuchsia', 'openbsd']
+    return ['linux', 'darwin', 'msvc', 'aix', 'fuchsia', 'freebsd', 'openbsd']
 
   def platform(self):
     return self._platform

util/exe_path.cc

diff --git a/util/exe_path.cc b/util/exe_path.cc
index 2f80d5a..3c9c9de 100644
--- a/util/exe_path.cc
+++ b/util/exe_path.cc
@@ -14,6 +14,7 @@
 #elif defined(OS_WIN)
 #include <windows.h>
 #elif defined(OS_FREEBSD)
+#include <limits.h>
 #include <sys/sysctl.h>
 #include <sys/types.h>
 #endif