tools/power: turbostat: make Makefile a bit more capable

The turbostat Makefile is pretty simple, its output is placed in the
same directory as the source, the install rule has no concept of a
prefix or sysroot, and you can set CC to use a specific compiler but
not use the more familiar CROSS_COMPILE. By making a few minor changes
these limitations are removed while leaving the default behavior
matching what it used to be.

Example build with these changes:
make CROSS_COMPILE=i686-wrs-linux-gnu- DESTDIR=/tmp install

or from the tools directory
make CROSS_COMPILE=i686-wrs-linux-gnu- DESTDIR=/tmp turbostat_install

Signed-off-by: Mark Asselstine <mark.asselstine@windriver.com>
Signed-off-by: Len Brown <len.brown@intel.com>