paclage/ace: explain why we use C++ pre-processor flags for C++ flags

Commit e8011a08b6fd (package/ace: needs C++11) did not explain why it
passed C++ flags via the pre-processor flags rather than as C++ flags
(via ACE's CCFLAGS).

This caused some head-scratching when reviewing and applying
803247337d35 (package/ace: fix build failure due to gcc bug 101915).

Add a comment to try and explain the non-standard use of pre-processor
flags to pass actual C++ flags.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Giulio Benetti <giulio.benetti@benettiengineering.com>
Cc; Weber, Matthew L Collins <Matthew.Weber@collins.com>
Cc: Fabrice Fontaine <fontaine.fabrice@gmail.com>
(cherry picked from commit b717880f1fb7728648a2426dc20c3985fa3c530b)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/ace/ace.mk

@@ -23,6 +23,9 @@ ifeq ($(BR2_TOOLCHAIN_HAS_GCC_BUG_101915),y)
 ACE_CPPFLAGS += -O0
 endif
 
+# ACE uses DEFFLAGS as C++ pre-processor flags, and CCFLAGS as the C++ flags.
+# Ace passes the pre-processor flags after the C++ flags, so we pass our
+# C++ flags as pre-processor flags, via DEFFLAGS.
 ACE_MAKE_OPTS = \
 	ACE_ROOT="$(@D)" \
 	DEFFLAGS="$(ACE_CPPFLAGS)"