do_cmake.sh: Use value of ARGS at runtime

The "Building Ceph" section of README.md instructs the user to run:

    ARGS="-DCMAKE_C_COMPILER=gcc-7" ./do_cmake.sh

But, very early on do_cmake.sh was doing ARGS="-GNinja", ignoring the existing
 value.

Signed-off-by: Zack Cerza <[email protected]>

Author: zmc

do_cmake.sh

@@ -14,7 +14,7 @@ if [ -e $BUILD_DIR ]; then
 fi
 
 PYBUILD="3"
-ARGS="-GNinja"
+ARGS="${ARGS} -GNinja"
 if [ -r /etc/os-release ]; then
   source /etc/os-release
   case "$ID" in