add option to disable installing g3log for project embedding

[bmagistro]

Based on API.markdown pointing people to Options.cmake no additional documentation was added.

Feature adds the option to skip installing components making it easier to embed this project using cmake's ExternalProject or FetchProject.

The change was modeled after the googletest options.
https://github.com/google/googletest/blob/306f3754a71d6d1ac644681d3544d06744914228/CMakeLists.txt#L30
https://github.com/google/googletest/blob/8c91ecef292e963d23cd5b25f01ea1579fbe9aaa/googletest/CMakeLists.txt#L96

Sample usage (has additional options enabled)

include(FetchContent)

# g3log
set(ADD_FATAL_EXAMPLE OFF CACHE BOOL "g3log examples" FORCE)
set(USE_DYNAMIC_LOGGING_LEVELS ON CACHE BOOL "g3log custom level support" FORCE)
set(G3_SHARED_LIB OFF CACHE BOOL "g3log static lib" FORCE)
set(INSTALL_G3LOG OFF CACHE BOOL "g3log install" FORCE)

FetchContent_Declare(
  g3log
  GIT_REPOSITORY https://github.com/KjellKod/g3log.git
  GIT_TAG        1.3.2
)

FetchContent_GetProperties(g3log)
if(NOT g3log_POPULATED)
  FetchContent_Populate(g3log)
  add_subdirectory(${g3log_SOURCE_DIR} ${g3log_BINARY_DIR})
endif()
# g3log

[KjellKod]

Looks good. Will verify manually just in case later today