Added cmake-stamp

[radarhere]

All build_* functions in library_builders.sh create a *-stamp file, to prevent the function from being run more than once. For example,

multibuild/library_builders.sh

Lines 155 to 163 in 9a9d127

	function build_jpeg {
	if [ -e jpeg-stamp ]; then return; fi
	fetch_unpack http://ijg.org/files/jpegsrc.v${JPEG_VERSION}.tar.gz
	(cd jpeg-${JPEG_VERSION} \
	&& ./configure --prefix=$BUILD_PREFIX \
	&& make -j4 \
	&& make install)
	touch jpeg-stamp
	}

This PR suggests adding that feature to get_modern_cmake as well. It is slightly different - all of the code within should not be skipped on subsequent calls, as get_modern_cmake does echo a result - but the install commands can be skipped.

[radarhere]

Superseded by #542