build: Allow disabling documentation build + print more build flags

[nerzhul]

I currently had a doxygen setup issue on my side, for unknown reason anyway.

It triggers issue on my side like this

minetest-stock/cmake-build-release on  master [$!] via △ v3.27.7 took 50s 
❯ cmake .. -D RUN_IN_PLACE=1
-- *** Will build version 5.8.0-dev ***
-- Using user-provided IrrlichtMt at subdirectory 'lib/irrlichtmt'
-- *** Building IrrlichtMt 1.9.0.12 ***
-- Performing Test REVISION_SANITY_CHECK
-- Performing Test REVISION_SANITY_CHECK - Success
-- Device: X11
-- OpenGL: ON
-- OpenGL 3: FALSE
-- OpenGL ES: OFF
-- OpenGL ES 2: OFF
-- WebGL: OFF
-- Found IrrlichtMt 1.9.0.12
-- Using GMP provided by system.
-- Using bundled JsonCpp library.
-- LuaJIT detection disabled! (ENABLE_LUAJIT=0)
-- LuaJIT not found, using bundled Lua.
-- cURL support enabled.
-- GetText enabled; locales found: ar;be;bg;ca;cs;cy;da;de;dv;el;eo;es;et;eu;fi;fil;fr;gd;gl;he;hi;hu;id;it;ja;jbo;kk;kn;ko;ky;lt;lv;lzh;mn;mr;ms;ms_Arab;nb;nl;nn;oc;pl;pt;pt_BR;ro;ru;sk;sl;sr_Cyrl;sr_Latn;sv;sw;th;tr;tt;uk;vi;yue;zh_CN;zh_TW
-- Sound enabled.
-- ncurses console enabled.
-- PostgreSQL backend enabled
-- PostgreSQL includes: /usr/include;/usr/include/postgresql/server
-- LevelDB not found!
-- Redis backend enabled.
-- Prometheus client disabled.
-- SpatialIndex AreaStore backend enabled.
-- Looking for ZSTD_initCStream
-- Looking for ZSTD_initCStream - found
-- Locale blacklist applied; Locales used: be;bg;ca;cs;cy;da;de;el;eo;es;et;eu;fi;fil;fr;gd;gl;hu;id;it;ja;jbo;kk;ko;ky;lt;lv;lzh;mn;mr;ms;nb;nl;nn;oc;pl;pt;pt_BR;ro;ru;sk;sl;sr_Cyrl;sr_Latn;sv;sw;tr;tt;uk;vi;yue;zh_CN;zh_TW
-- Performing Test HAVE_ATCCALL
-- Performing Test HAVE_ATCCALL - Success
CMake Warning at /usr/share/cmake/Modules/FindDoxygen.cmake:497 (message):
  Unable to determine doxygen version: No such file or directory
Call Stack (most recent call first):
  /usr/share/cmake/Modules/FindDoxygen.cmake:660 (_Doxygen_find_doxygen)
  CMakeLists.txt:399 (find_package)


CMake Error at /usr/share/cmake/Modules/FindDoxygen.cmake:739 (message):
  Unable to generate Doxyfile template: No such file or directory
Call Stack (most recent call first):
  CMakeLists.txt:399 (find_package)

I added a cmake build flag (BUILD_DOCUMENTATION) to disable documentation build (default is enabled)

I also added output for some interesting flags, to remember what targets we wanted to build

❯ cmake .. -D RUN_IN_PLACE=TRUE -DBUILD_DOCUMENTATION=FALSE
-- *** Will build version 5.8.0-dev ***
-- Compiling client: TRUE
-- Compiling server: FALSE
-- Compiling unittests: TRUE
-- Compiling benchmarks: FALSE
-- Generating documentation: FALSE
-- Run in place build: TRUE
-- Using user-provided IrrlichtMt at subdirectory 'lib/irrlichtmt'
-- *** Building IrrlichtMt 1.9.0.12 ***

[nerzhul]

@grorp thanks for your review, fixes are now adressed