build: Remove "-I$(top_srcdir)/intl" from CPPFLAGS.

build: Remove "-I$(top_srcdir)/intl" from CPPFLAGS.

Revert commit e02c3d27eeb6b53ec82532ab080c23ce26813fd4

("include the intl/ subdirectory when searching for include files")

Flex doesn't come with libintl source code, so inclusion of
$(top_srcdir)/intl subdirectory during compiling is unnecessary.

This flag seems to be left after "remove intl from dist"
(commit 8f45da06adbd7dbcdfdbdd5c20ff4c47fc504f35) without someone
cleaning this up.

Users who need to build with gettext functionality from libintl would
use the '--with-libintl-prefix' configure option.