diff --git a/gitlab/functions.sh b/gitlab/functions.sh
index 6a9b28f94e0ccc1b73c6fb8f014e2a936b59c9e1..cbfea14058d14af95b9d19e6d176b60b8643f30e 100644
--- a/gitlab/functions.sh
+++ b/gitlab/functions.sh
@@ -203,6 +203,18 @@ prepare_build_env() {
 }
 
 
+# Appends a value to a variable if that is not already set
+# $1: variable name to append the value
+# $2: the value to be appended
+append_if_not_set() {
+  if [ -z "${1+abc}" ]; then
+    eval $1="$2"
+  elif [[ "${!1}" != "*${2}*" ]]; then
+    eval $1="${!1} $2"
+  fi
+}
+
+
 # Checks if an array contains a value
 # taken from here: https://stackoverflow.com/questions/3685970/check-if-an-array-contains-a-value
 # Parameters: <value-to-check> <array-variable>
@@ -300,12 +312,12 @@ else
 fi
 
 if [ "${OSNAME}" == "macosx" ]; then
+  CC=gcc
+  export_env CC
+  CXX=g++
+  export_env CXX
   MACOSX_DEPLOYMENT_TARGET="10.7"
   export_env MACOSX_DEPLOYMENT_TARGET
-  if [ -n "${CPPFLAGS}" ]; then
-    CPPFLAGS="${CPPFLAGS} -DBOOST_NO_CXX11_RVALUE_REFERENCES"
-  else
-    CPPFLAGS="-DBOOST_NO_CXX11_RVALUE_REFERENCES"
-  fi
+  append_if_not_set CPPFLAGS "-DBOOST_NO_CXX11_RVALUE_REFERENCES"
   export_env CPPFLAGS
 fi