Update BUILD scripts

- Remove -Wimplicit-fallthrough=2 for gcc versions < 6
- Don't do git submodule update one fresh git clones
  This fixes an issue when using git 1.0 that gives
  errors on empty submodule directories

BUILD/FINISH.sh

@@ -34,7 +34,7 @@ fi
 
 commands=""
 # git clean -fdX removes all ignored (build) files
-if test -d .git
+if test -d .git && test -d libmariadb/CMakeFiles
 then
     commands="\
 git clean -fdX

BUILD/SETUP.sh

@@ -145,7 +145,7 @@ elif [ "x$warning_mode" = "xmaintainer" ]; then
   debug_extra_cflags="-g3"
 else
 # Both C and C++ warnings
-  warnings="-Wall -Wextra -Wunused -Wwrite-strings -Wno-uninitialized -Wno-strict-aliasing -Wimplicit-fallthrough=2 -Wformat-security -Wvla"
+  warnings="-Wall -Wextra -Wunused -Wwrite-strings -Wno-uninitialized -Wno-strict-aliasing -Wformat-security -Wvla"
 
 # For more warnings, uncomment the following line
 # warnings="$warnings -Wshadow"