use CPPFLAGS instead of CXXFLAGS for stringview export

Git-Dch: Ignore

apt-pkg/makefile

@@ -12,7 +12,7 @@ include ../buildlib/defaults.mak
 # The library name and version (indirectly used from init.h)
 include ../buildlib/libversion.mak
 
-CXXFLAGS+=-DAPT_PKG_EXPOSE_STRING_VIEW
+CPPFLAGS+=-DAPT_PKG_EXPOSE_STRING_VIEW
 LIBRARY=apt-pkg
 MAJOR=$(LIBAPTPKG_MAJOR)
 MINOR=$(LIBAPTPKG_RELEASE)