build: Remove unused -I options from AM_CPPFLAGS.

build: Remove unused -I options from AM_CPPFLAGS.

* gnulib-local/modules/gettext-runtime-misc (Makefile.am): Remove
-I$(top_srcdir)/intl from AM_CPPFLAGS.
* gettext-runtime/tests/Makefile.am (AM_CPPFLAGS): Set to empty.
* gettext-tools/gnulib-lib/Makefile.am (AM_CPPFLAGS): Remove
-I$(top_srcdir)/../gettext-runtime/intl.
* gettext-tools/gnulib-tests/Makefile.am (AM_CPPFLAGS): Update comment.
* gettext-tools/src/Makefile.am (AM_CPPFLAGS): Add comment.
* gettext-tools/libgettextpo/Makefile.am (AM_CPPFLAGS): Likewise.
* libtextstyle/lib/Makefile.am (AM_CPPFLAGS): Remove -I$(top_srcdir).