full cmake 5/N: prerequisites

[loriab]

currently atop #313 . diff of the new stuff: loriab/libint@configconfig...loriab:libint:l2deps

• handle gmp/mpfr, boost, eigen, python deps
• fetchcontent-advocating ppl have convinced me that L2-only library options should probably start with LIBINT2_. And if options add functionality, I've added ENABLE. All changes documented in the upgrade guide.
• gmp/mpfr
  • (copied from 2022 PR) rename FindMPFR.cmake to FindMultiprecision.cmake to handle gmp, gmpxx, and mpfr targets. Based on offical cmake module FindGSL.cmake to hopefully handle Windows, too. This way build_libint seeks gmpxx component, and the library (if mprfr enabled) seeks gmpxx and mpfr.
• Boost
  • I think I saw some Boost >=1.29. I switched everything to minimum 1.57 (c.2014)
  • The Boost handling logic is simplified some. Your try_compile() sample code is tucked away in a cmake include. The specialty USE_BUNDLED_BOOST is replaced by the standard DISABLE_FIND_PACKAGE_Boost. If the sample code fails on a detected Boost, the build stops, rather than continuing to the bundled boost.
• Eigen
  • (copied from 2022 PR) reworked Eigen3 detection. copied a FindEigen3.cmake from latest Eigen release. This is installed with L2 and the local path is prepended so this FindEigen3 is always used. The FindEigen already handles (1) finding installations with a Eigen3Config.cmake, (2) seeking if the Eigen3_INCLUDE_DIR already in cache, (3) searching for header like a traditional FindPkg.cmake. It always returns a target, Eigen3::Eigen. This has been modified to address some of your concerns. (1) turned off build package registry to avoid the ephemeral build trees. (2) If an option LIBINT_LOCAL_Eigen3_FIND is set, the FindEigen instead looks to import a local L2-defined target (see below) to ensure ABI consistency. This is off by default. The FindEigen3 detection is shared by the library build and libint2-config.cmake
  • (copied from 2022 PR) Eigen3 handling on the src/lib/libint/CM.export side is outsourced to FindEigen3. If LIBINT_LOCAL_Eigen3_INSTALL is set, the detected eigen3 is installed as a target in a separate file. This is off by default, but I don't care if it's on by default, so long as it's turn-off-able and the _FIND option is off by default.
  • As far as I can see, Eigen is irrelevant to the generator/compiler, so I didn't detect and include it for config.h. I figured it was there for libtool access.
  • There's an eigen test code in lib/autoconf/acx_check_eigen.m4 . Do you want that run on the cmake-detected eigen?
• Python
  • Dependency pattern unchanged and already added Fall 2023.