contrib: Fix default variable value syntax

Change-Id: I23abf33881f64dd89c0bf5ef970972908d337366
diff --git a/contrib/jenkins-build-common.sh b/contrib/jenkins-build-common.sh
index d64f92d..ca8bf02 100644
--- a/contrib/jenkins-build-common.sh
+++ b/contrib/jenkins-build-common.sh
@@ -42,16 +42,16 @@
   exit 1
 fi
 
-git_url="${git_url-"git://git.osmocom.org"}"
-prefix="${prefix-"$base/inst-$name"}"
+git_url="${git_url:-git://git.osmocom.org}"
+prefix="${prefix:-$base/inst-$name}"
 # prefix_real is usually identical with prefix, except when installing to a
 # different $DESTDIR than /, which is the case for example when building
 # osmo-bts within the sysmoBTS SDK
-prefix_real="${prefix_real-"$prefix"}"
+prefix_real="${prefix_real:-$prefix}"
 
 # Flag to be used to enable ASAN in builds. Defaults to enable ASAN builds and
 # it can be disabled by passing SANITIZE_FLAGS="" to the build.
-SANITIZE_FLAGS="${SANITIZE_FLAGS---enable-sanitize}"
+SANITIZE_FLAGS="${SANITIZE_FLAGS:---enable-sanitize}"
 
 export PKG_CONFIG_PATH="$prefix_real/lib/pkgconfig:$PKG_CONFIG_PATH"
 export LD_LIBRARY_PATH="$prefix_real/lib:$LD_LIBRARY_PATH"