#186 support gettext 0.20

[CvH]

support compiling if gettext 0.20 or later is found

AM_GNU_GETTEXT_REQUIRE_VERSION overwrites AM_GNU_GETTEXT_VERSION if autoreconf supports it, so legacy systems are still supported

[CvH]

fixes
*** error: gettext infrastructure mismatch: using a Makefile.in.in from gettext version 0.18 but the autoconf macros are from gettext version 0.20