build: range-v3 dependency added to CMake config file for clang + libc++

src/mp-unitsConfig.cmake

@@ -20,8 +20,35 @@
 # OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 # SOFTWARE.
 
+function(__check_libcxx_in_use variable)
+    if(CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
+        message(CHECK_START "Checking if libc++ is being used")
+        list(APPEND CMAKE_MESSAGE_INDENT "  ")
+
+        include(CheckSymbolExists)
+        check_symbol_exists(_LIBCPP_VERSION "ciso646" ${variable})
+        set(${variable} ${${variable}} PARENT_SCOPE)
+
+        list(POP_BACK CMAKE_MESSAGE_INDENT)
+        if(UNITS_LIBCXX)
+            message(CHECK_PASS "found")
+        else()
+            message(CHECK_FAIL "not found")
+        endif()
+    endif()
+endfunction()
+
 include(CMakeFindDependencyMacro)
 find_dependency(fmt)
 find_dependency(gsl-lite)
 
+# add range-v3 dependency only for clang + libc++
+if(CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
+    __check_libcxx_in_use(__units_libcxx)
+    if(__units_libcxx)
+        find_dependency(range-v3)
+    endif()
+    unset(__units_libcxx)
+endif()
+
 include("${CMAKE_CURRENT_LIST_DIR}/mp-unitsTargets.cmake")