extract travis installs from build-depends automatically

I forgot to add libgtest-dev to the list of packages to install on
travis, so this slightly hacky oneliner might prevent us from having
the same problem again if we happen to change dependencies again.

Git-Dch: Ignore

.travis.yml

@@ -1,5 +1,6 @@
 language: cpp
 before_install:
  - sudo apt-get update -q
- - sudo apt-get install -q --no-install-recommends dpkg-dev debhelper libdb-dev gettext libcurl4-gnutls-dev zlib1g-dev libbz2-dev xsltproc docbook-xsl docbook-xml po4a autotools-dev autoconf automake doxygen debiandoc-sgml stunnel4
+ - sudo ./prepare-release travis-ci
+ - sudo apt-get install -q --no-install-recommends stunnel4
 script: make && make test && test/integration/run-tests

prepare-release

@@ -2,7 +2,6 @@
 set -e
 
 cd "$(readlink -f $(dirname $0))"
-dpkg-checkbuilddeps -d 'libxml2-utils'
 
 if [ -n "${GBP_BUILD_DIR}" ]; then
    cd "$GBP_BUILD_DIR"
@@ -68,6 +67,8 @@ elif [ "$1" = 'post-build' ]; then
 		echo >&2 'REMEMBER: Change to a valid distribution before release'
 	fi
 
+	dpkg-checkbuilddeps -d 'libxml2-utils'
+
 	# check the manpages with each vendor for vendor-specific errors…
 	find vendor -mindepth 1 -maxdepth 1 -type d | cut -d'/' -f 2 | while read DISTRO; do
 		ln -sf ../vendor/${DISTRO}/apt-vendor.ent doc
@@ -102,6 +103,8 @@ elif [ "$1" = 'buildlog' ]; then
 		librarysymbolsfromfile "$2"
 		shift
 	done
+elif [ "$1" = 'travis-ci' ]; then
+	apt-get install -q --no-install-recommends $(sed -n -e '/^Build-Depends: /,/^Build-Depends-Indep: / {p}' debian/control | sed -e 's#([^)]*)##g' -e 's#^Build-Depends\(-Indep\)\?: ##' | tr -d ',')
 else
 	echo >&1 "Usage:\t$0 pre-export
 \t$0 post-build