make_build_h: handle backslashes in external strings

LikewiseOpen creates user names with a backslash in them. A gentle
massage with sed(1) allows such strings to be inserted into build.h
properly quoted. I am also adding the same for host name and other
strings used in the script that are more or less user-controlled.

ASTERISK-26754

Change-Id: Iac5ef2b67a68ee58f35ddbf86bb818ba6eabecae

build_tools/make_build_h

 #!/bin/sh
-HOSTNAME=`uname -n`
-KERNEL=`uname -r`
-MACHINE=`uname -m`
+HOSTNAME=`uname -n | sed 's/\\\\/\\\\\\\\/g'`
+KERNEL=`uname -r   | sed 's/\\\\/\\\\\\\\/g'`
+MACHINE=`uname -m  | sed 's/\\\\/\\\\\\\\/g'`
 OS=`uname -s`
-USER=`id | awk -F")" '{print $1}'| awk -F"(" '{print $2}'`
+USER=`id | awk -F")" '{print $1}'| awk -F"(" '{print $2}' | sed 's/\\\\/\\\\\\\\/g'`
 DATE=`date -u "+%Y-%m-%d %H:%M:%S"`
 cat << END
 /*
- * build.h 
+ * build.h
  * Automatically generated
  */
 #define BUILD_HOSTNAME "${HOSTNAME}"