cmake

Fix CMake install prefix when not set at configure time #568

Job	Run time
Install Ubuntu GCC static Release	14s
Install Ubuntu GCC header-only Release	14s
Install Ubuntu GCC shared Release	16s
Ubuntu GCC Release	1m 57s
Ubuntu GCC Debug	1m 8s
Ubuntu GCC noexcept Release	1m 4s
Ubuntu GCC noexcept Debug	42s
Ubuntu GCC notime Release	1m 11s
Ubuntu GCC notime Debug	45s
Ubuntu GCC nounity Release	1m 17s
Ubuntu GCC nounity Debug	55s
Ubuntu GCC header-only Release	1m 23s
Ubuntu GCC header-only Debug	53s
Ubuntu GCC shared Release	1m 19s
Ubuntu GCC shared Debug	47s
Ubuntu Clang Release	1m 16s
Ubuntu Clang Debug	55s
Ubuntu Clang noexcept Release	1m 50s
Ubuntu Clang noexcept Debug	47s
Windows 32 Release	2m 45s
Windows 32 Debug	1m 58s
Windows 64 Release	2m 53s
Windows 64 Debug	1m 28s
Windows 64 noexcept Release	3m 10s
Windows 64 noexcept Debug	1m 55s
Windows 64 shared Release	3m 11s
Windows 64 shared Debug	2m 1s
MacOS Release	40s
MacOS Debug	33s
MacOS noexcept Release	36s
MacOS noexcept Debug	29s
WebAssembly Release	4m 5s
WebAssembly Debug	1m 38s
WebAssembly noexcept Release	1m 28s
WebAssembly noexcept Debug	3m 36s
	51m 19s

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