feat(cmake): pass-through the custom C++ namespace from CMake (#15)

Via defining the LB_STD_VERSION_NS macro using the
target_compile_definitions().
Its name a bit inconsistent though, but looks better (in my opinion) in
C++, but doesn't look OK in a bunch of CMake configure parameters as it
conflicts with another flags (`LBSTD_VERSION_xxx` vs `LB_STD_VERSION_xxx`).

CMakeLists.txt

@@ -5,17 +5,22 @@
 cmake_minimum_required(VERSION 3.20)
 project(lbstd_version VERSION 0.0.1 LANGUAGES CXX)
 
-option(LBSTD_VERSION_NS_NAME "The enclosing namespace for all classes of std_version" "lbstd")
+set(LBSTD_VERSION_NS "lbstd" CACHE STRING "The enclosing namespace for all classes of std_version")
 option(LBSTD_VERSION_ENABLE_FMTLIB "Enable fmtlib formatters integration" ON)
 option(LBSTD_VERSION_ENABLE_STDFORMAT "Enable std::format integration")
 option(LBSTD_VERSION_ENABLE_TESTS "Enable tests" ON)
 
+if (LBSTD_VERSION_NS STREQUAL "")
+  message(FATAL_ERROR "The enclosing namespace name in variable LBSTD_VERSION_NS must be a non-empty string!")
+endif()
+
 if (LBSTD_VERSION_ENABLE_FMTLIB)
   find_package(fmt REQUIRED)
 endif()
 
 add_library(lbstd_version INTERFACE)
 target_include_directories(lbstd_version INTERFACE "${CMAKE_SOURCE_DIR}/include")
+target_compile_definitions(lbstd_version INTERFACE "LB_STD_VERSION_NS=${LBSTD_VERSION_NS}")
 target_compile_features(lbstd_version INTERFACE cxx_std_20)
 
 if (LBSTD_VERSION_ENABLE_TESTS)