cmake: Implement install build target

CMakeLists.txt

@@ -187,6 +187,8 @@ option(BUILD_BENCH "Build bench_bitcoin executable." OFF)
 option(BUILD_FUZZ_BINARY "Build fuzz binary." OFF)
 cmake_dependent_option(BUILD_FOR_FUZZING "Build for fuzzing. Enabling this will disable all other targets and override BUILD_FUZZ_BINARY." OFF "NOT MSVC" OFF)
 
+option(INSTALL_MAN "Install man pages." ON)
+
 set(configure_warnings)
 
 include(CheckPIESupported)