specialfunctions: generalize `gamma` precision #3526

Job	Run time
Build (ubuntu-latest, gcc, 10, cmake)	2m 4s
Build (ubuntu-latest, gcc, 13, cmake)	1m 55s
Build (ubuntu-latest, gcc, 11, cmake)	2m 12s
Build (ubuntu-22.04, cmake, intel-classic, 2021.10)	6m 24s
Build (macos-13, gcc, 11, cmake)	4m 25s
Build (macos-13, gcc, 10, cmake)	5m 51s
Build (macos-13, gcc, 12, cmake)	4m 49s
Build (ubuntu-latest, gcc, 12, cmake)	2m 7s
Build (ubuntu-latest, intel, 2024.1, cmake)	5m 45s
Build (ubuntu-latest, cmake-inline, gcc, 10)	2m 28s
	38m 0s

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