This grew out of my work creating an sfml port for macports, but should
be helpful for package maintainers of various distributions:
* add an SFML_USE_SYSTEM_DEPS option to ignore everything in extlibs/
except for headers/stb_image, and use the system versions
* install pkg-config files if a pkg-config program is found
and either lib/pkgconfig or libdata/pkgconfig exists under the
INSTALL_PREFIX, or the SFML_INSTALL_PKGCONFIG_FILES flag is set
explicitly
* install pkg-config files for static libs too, add the necessary
Requires.private and Libs.private entries to the .pc files to support
static linking
* on OS X, honor all INSTALL_NAME and RPATH related cmake variables and
only set the INSTALL_NAME_DIR to "@rpath" if none of them is set, this
preserves the default behavior of using @rpath but also allows
overriding by the usual cmake mechanisms
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