Changes in version 7.3:

• util.cpp: Tune gourdon alpha factors for primecount-7.3.
• nth_prime.cpp: Improved performance for small n.
• FactorTable.hpp: Reduce memory allocations.
• DFactorTable.hpp: Reduce memory allocations.
• S2_trivial.cpp: Reduce memory allocations.
• SegmentedPiTable.cpp: Reduce memory allocations.
• CMakeLists.txt: Detect at build time if the C++ compiler and the C++ Standard Library support int128_t. If not, int128_t will be disabled. The -std=c++* option usually disables int128_t, use -std=gnu++* instead (or omit both -std=c++* and -std=gnu++*).
• CMakeLists.txt: Detect at build time if the host CPU (x86) supports the POPCNT instruction. If not, disable the POPCNT instruction. (NOTE: Fedora unconditionally disables POPCNT to support older CPUs.)
• CMakeLists.txt: Fix AppleClang OpenMP detection.
• CMakeLists.txt: Print warning message if OpenMP library is missing.
• CMakeLists.txt: Add option STATICALLY_LINK_LIBPRIMECOUNT=ON/OFF.
• Update to latest libprimesieve-7.9.
• appveyor.yml: Test primecount using many different C++ compilers: GCC-5, GCC-7, GCC-8, GCC-9, Clang-9, AppleClang 13, MSVC 2019.
• ci.yml: New Github Actions tests: Windows/MinGW-w64 and Linux with Valgrind and PrimePi(10^20) 128-bit test.