easy-builder: make it compatible with Ubuntu 18.04/CMake 3.10

on CMake 3.10i, options -S and -B were not supported yet

diff --git a/easy-builder b/easy-builder
index 39857382c5ddad6fd2fe1c07cb0ab99f11cbff55..82c08d90836a84dc99d4b832d77f780d554bceff 100755 (executable)
--- a/easy-builder
+++ b/easy-builder
@@ -81,10 +81,11 @@ set -x
 
 "${fetch_submodules_cmd[@]}"
 
+mkdir -pv "${build_dir}"
+cd "${build_dir}"
+
 cmake \
        -G'Unix Makefiles' \
-       -S"${project_source_dir}" \
-       -B"${build_dir}" \
        -D'CMAKE_INSTALL_PREFIX'="${install_dir}" \
        -D'CMAKE_BUILD_TYPE'="${build_type}" \
        "${cmake_user_opts[@]}" \