build: Remove redundant -I options from AM_CPPFLAGS.
https://www.gnu.org/software/automake/manual/html_node/Program-Variables.html
documents that -I. -I$(srcdir) and -I$(top_builddir) (for config.h)
are already part of Automake's $(DEFAULT_INCLUDES), which is used
before $(AM_CPPFLAGS).
* gettext-runtime/src/Makefile.am (AM_CPPFLAGS): Remove redundant -I options.
* gettext-tools/gnulib-lib/Makefile.am (AM_CPPFLAGS): Likewise.
* gettext-tools/libgettextpo/Makefile.am (AM_CPPFLAGS): Likewise.
* gettext-tools/libgrep/Makefile.am (AM_CPPFLAGS): Likewise.
* gettext-tools/src/Makefile.am (AM_CPPFLAGS): Likewise.
* gettext-tools/system-tests/Makefile.am (AM_CPPFLAGS): Likewise.
* gettext-tools/tests/Makefile.am (AM_CPPFLAGS): Likewise.
* libtextstyle/adhoc-tests/Makefile.am (AM_CPPFLAGS): Likewise.
* libtextstyle/lib/Makefile.am (AM_CPPFLAGS): Likewise.
* gettext-runtime/intl/Makefile.am (AM_CPPFLAGS): Add comment.
* gettext-runtime/libasprintf/Makefile.am (AM_CPPFLAGS): Likewise.
