build: do not require xsltproc when building from a release tarball

Fixes https://gitlab.freedesktop.org/mobile-broadband/ModemManager/-/issues/276

configure.ac

@@ -49,9 +49,6 @@ AM_PROG_CC_C_O
 AC_PROG_INSTALL
 AC_PROG_MKDIR_P
 
-AC_CHECK_PROG(XSLTPROC_CHECK,xsltproc,yes)
-AS_IF([test x"$XSLTPROC_CHECK" != x"yes"], [AC_MSG_ERROR([Please install xsltproc before configuring.])])
-
 dnl Initialize libtool
 LT_PREREQ([2.2])
 LT_INIT([disable-static])
@@ -165,6 +162,12 @@ AC_SUBST(GLIB_MKENUMS)
 GDBUS_CODEGEN=`$PKG_CONFIG --variable=gdbus_codegen gio-2.0`
 AC_SUBST(GDBUS_CODEGEN)
 
+dnl xsltproc required in git builds only
+AC_CHECK_PROG(XSLTPROC_CHECK,xsltproc,yes)
+if test "x$ax_is_release" != "xyes" -a "x$XSLTPROC_CHECK" != "xyes"; then
+   AC_MSG_ERROR([Please install xsltproc before configuring.])
+fi
+
 dnl-----------------------------------------------------------------------------
 dnl Testing support
 dnl