Improve install with exported options

[eddyashton]

Resolve items listed in #830.

• I've renamed our TARGET option to COMPILE_TARGETS, to distinguish it from all the other uses of TARGET in cmake
• I've used add_compile_definitions rather than add_definitions, for no precise benefit
• I've removed SERVICE_IDENTITY_CURVE definitions. If a user wants CCF itself to use a faster curve, they can change one line of code. But the installed headers as seen by the libccf build and the app build should match, without requiring too many obscure preprocessor defines

[ghost]

install_export_more@6426 aka 20200325.27 vs master ewma over 30 builds from 6053 to 6422