Add scripts for automating more parts of the release process

Update dates in various files before the release automatically.

Also automatically update SHA-1 of the release files both before the
release (to zero them) and afterwards (to use the correct values).

Closes https://github.com/wxWidgets/wxWidgets/pull/443

build/tools/pre-release.sh

@@ -0,0 +1,58 @@
+#!/bin/sh
+
+# build a list of English locales
+declare -a locale_list=("en_GB" "en_US")
+# get available en locales (including .utf8 and .UTF-8 ones)
+declare -a en_locales=(`locale -a | grep '^C\|^POSIX\|^en_'`)
+locale_list+=("${en_locales[@]}")
+
+found=0
+
+# set an English locale to get the month name
+for i in "${locale_list[@]}"
+do
+    LC_ALL="$i"
+
+    # check that setting the locale succeeded
+    # if it failed then LC_TIME would be empty
+    my_locale=`locale | grep LC_TIME | sed 's/.*"\(.*\)".*/\1/'`
+    if [ "$my_locale" = "$i" ]
+    then
+        echo Locale set to $i
+        found=1
+        break
+    fi
+done
+
+if [ $found = 0 ]
+then
+    echo Could not set an appropriate locale
+    exit 1
+fi
+
+# Exit immediately if a command exits with a non-zero status.
+set -e
+
+# get the month name and the year, see $(date +'%B %Y')
+# then use it in the relevant line (one before last)
+sed -i "/^The wxWidgets Team, / s/.*/The wxWidgets Team, $(date +'%B %Y')/" ./../../docs/readme.txt
+echo Updated date in docs/readme.txt
+
+# reset checksums to a string of 40 0-s, see $(printf '%.40d' 0)
+sed -i -E "/^\s*[0-9a-f]{40}\s{1,2}wx/ s/(\s*)[0-9a-f]{40}(\s{1,2}wx)/\1$(printf '%.40d' 0)\2/" ./../../docs/release.md
+echo Reset checksums in docs/release.md
+
+# get current date, see $(date +'%Y-%m-%d')
+# then use it in the line containing "X.X.X: (not released yet)"
+sed -i "/^[3-9]\.[0-9]\.[0-9]: (not released yet)$/ s/not released yet/released $(date +'%Y-%m-%d')/" ./../../docs/changes.txt
+echo Updated release date in docs/changes.txt
+
+# get current date, see $(date +'%B %d, %Y')
+# then use it in the line starting with @date
+sed -i "/^@date / s/.*/@date $(date +'%B %d, %Y')/" ./../../docs/doxygen/mainpages/manual.h
+echo Updated date in docs/doxygen/mainpages/manual.h
+
+# get current date, see $(date +'%B %d, %Y')
+# then use it in the first line
+sed -i "1s/^.* -- /$(date +'%B %d, %Y') -- /" ./../../docs/publicity/announce.txt
+echo Updated date in docs/publicity/announce.txt