Fix for non-portable GFortran -J flag in install script (#692)

Author: wyphan

install.sh

@@ -64,7 +64,10 @@ fi
 
 $FETCH $SOURCE_URL > $BOOTSTRAP_DIR/fpm.F90
 
-$FC $FFLAGS -J $BOOTSTRAP_DIR $BOOTSTRAP_DIR/fpm.F90 -o $BOOTSTRAP_DIR/fpm
+SAVEDIR="$(pwd)"
+cd $BOOTSTRAP_DIR
+$FC $FFLAGS fpm.F90 -o fpm
+cd "$SAVEDIR"
 
 $BOOTSTRAP_DIR/fpm update
 $BOOTSTRAP_DIR/fpm install --compiler "$FC" --flag "$FFLAGS" --prefix "$PREFIX"