Include <picolibc.h> to detect _NEWLIB_VERSION

Author: ldionne

libcxx/include/__configuration/platform.h

@@ -42,6 +42,13 @@
 #  endif
 #endif
 
+// This is required in order for _NEWLIB_VERSION to be defined in places where we use it.
+// TODO: We shouldn't be including arbitrarily-named headers from libc++ since this can break valid
+//       user code. Move code paths that need _NEWLIB_VERSION to another customization mechanism.
+#if __has_include(<picolibc.h>)
+#  include <picolibc.h>
+#endif
+
 #ifndef __BYTE_ORDER__
 #  error                                                                                                               \
       "Your compiler doesn't seem to define __BYTE_ORDER__, which is required by libc++ to know the endianness of your target platform"

libcxx/test/configs/armv7m-picolibc-libc++.cfg.in

@@ -11,10 +11,6 @@ config.substitutions.append(('%{compile_flags}',
     # "large atomic operation may incur significant performance penalty; the
     # access size (4 bytes) exceeds the max lock-free size (0 bytes)"
     ' -Wno-atomic-alignment'
-
-    # Various libc++ headers check for the definition of _NEWLIB_VERSION
-    # which for picolibc is defined in picolibc.h.
-    ' -include picolibc.h'
 ))
 config.substitutions.append(('%{link_flags}',
     '-nostdlib -nostdlib++ -L %{lib-dir} -lc++ -lc++abi'