Fix ignores and add per-OS make_vars

.gitignore

@@ -1,6 +1,7 @@
 Makefile
 !third_party/libjpeg_turbo/yasm/source/config/linux/Makefile
 *.mk
+!install/**/*.mk
 *.Makefile
 client.timestamp
 hooks.timestamp

install/centos/make_vars.mk

@@ -0,0 +1,17 @@
+# Copyright 2016 Google Inc. All Rights Reserved.
+# Author: cheesy@google.com (Steve Hill)
+#
+# System-specific make vars for CentOS.
+
+APACHE_CONTROL_PROGRAM=/etc/init.d/httpd
+APACHE_LOG=/var/log/httpd/error_log
+APACHE_MODULES=/etc/httpd/modules
+APACHE_CONF_FILE=/etc/httpd/conf/httpd.conf
+APACHE_PIDFILE=/var/run/httpd.pid
+APACHE_PROGRAM=/usr/sbin/httpd
+APACHE_ROOT=/etc/httpd
+APACHE_STOP_COMMAND=stop
+APACHE_USER=apache
+BINDIR=/usr/local/bin
+SSL_CERT_DIR=/etc/pki/tls/certs
+SSL_CERT_FILE_COMMAND=ModPagespeedSslCertFile /etc/pki/tls/cert.pem

install/ubuntu/make_vars.mk

@@ -0,0 +1,20 @@
+# Copyright 2016 Google Inc. All Rights Reserved.
+# Author: cheesy@google.com (Steve Hill)
+#
+# System-specific make vars for Ubuntu.
+#
+# At the time of writing most of the Makefile defaults were actually for
+# Ubuntu, so this file is somewhat redundant.
+
+APACHE_CONTROL_PROGRAM=/etc/init.d/apache2
+APACHE_LOG=/var/log/apache2/error.log
+APACHE_MODULES=/usr/lib/apache2/modules
+APACHE_CONF_FILE=/etc/apache2/apache2.conf
+APACHE_PIDFILE=/var/run/apache2.pid
+APACHE_PROGRAM=/usr/sbin/apache2
+APACHE_ROOT=/etc/apache2
+APACHE_STOP_COMMAND=stop
+APACHE_USER=www-data
+BINDIR=/usr/local/bin
+SSL_CERT_DIR=/etc/ssl/certs
+SSL_CERT_FILE_COMMAND=