Always use LC_NUMERIC=C internally (#8204)

In most cases, like math, we want C-semantics for floating point
numbers. In particular "." needs to be the decimal separator.

Instead, we pay the price in printf, which is currently the sole place
to output in locale-specific numbers and attempt to read them and
C-style ones.

cmake/ConfigureChecks.cmake

@@ -146,6 +146,8 @@ endif()
 list(APPEND WCSTOD_L_INCLUDES "wchar.h")
 check_cxx_symbol_exists(wcstod_l "${WCSTOD_L_INCLUDES}" HAVE_WCSTOD_L)
 
+check_cxx_symbol_exists(uselocale "locale.h;xlocale.h" HAVE_USELOCALE)
+
 cmake_push_check_state()
 set(CMAKE_EXTRA_INCLUDE_FILES termios.h sys/ioctl.h)
 check_type_size("struct winsize" STRUCT_WINSIZE LANGUAGE CXX)