[build] do not output errors when pid_max does not exist

Summary: This was spamming the build output on osx.

Reviewed By: ezgicicek

Differential Revision: D23190091

fbshipit-source-id: 9ad33655f

Makefile.config

@@ -128,8 +128,12 @@ endif
 
 MAKE := $(MAKE) INTERACTIVE=$(INTERACTIVE)
 
-# 9999999 PIDs ought to be enough for anybody, but check if pid_max can be found just in case
+# check if pid_max can be found else default to whatever
-MAX_PID_SIZE = $(shell echo $$(( $$(wc -m < /proc/sys/kernel/pid_max 2>/dev/null) - 1 )) || echo 7)
+KERNEL_PID_MAX_FILE = /proc/sys/kernel/pid_max
+MAX_PID_SIZE = $(shell \
+    [ -r $(KERNEL_PID_MAX_FILE) ] \
+    && echo $$(( $$(wc -m < $(KERNEL_PID_MAX_FILE)) - 1 )) \
+ || echo 5)
 
 # Arguments:
 #   $(1) is a string describing the command