Make genrpm.sh a little safer. (#895)

1. Remove function `get_att_val`:
That function uses `eval` to set default values for bash variables.
Because it is very dangerous to use `eval` (someone call it the plague of shell scripting), I'd like to use the bash builtin to do that.
2. Double quote all variables to avoid unexpected expansion when using it.

rpm/genrpm.sh

@@ -38,28 +38,18 @@ do
     esac
 done
 
-get_att_val()
-{
-    att=$1
-    val=$2
-
-    if [ -z $(eval echo \$$att) ]; then
-        eval $att=$val
-    fi
-}
-
-get_att_val version "2.5.5"
-get_att_val format "tar.gz"
+: ${version:=2.5.5}
+: ${format:=tar.gz}
 
 name="shadowsocks-libev"
 spec_name="shadowsocks-libev.spec"
 
 pushd `git rev-parse --show-toplevel`
-git archive v${version} --format=${format} --prefix=${name}-${version}/ -o rpm/SOURCES/${name}-${version}.${format}
+git archive "v${version}" --format="${format}" --prefix="${name}-${version}/" -o rpm/SOURCES/"${name}-${version}.${format}"
 pushd rpm
 
 sed -e "s/^\(Version:	\).*$/\1${version}/" \
     -e "s/^\(Source0:	\).*$/\1${name}-${version}.${format}/" \
-    SPECS/${spec_name}.in > SPECS/${spec_name}
+    SPECS/"${spec_name}".in > SPECS/"${spec_name}"
 
-rpmbuild -bb SPECS/${spec_name} --define "%_topdir `pwd`"
+rpmbuild -bb SPECS/"${spec_name}" --define "%_topdir `pwd`"