Re: [PATCH] build: Fix build with gettext 0.20.1

[Jose E. Marchesi]

    Hi Eric.
    
        While the gnulib libtextstyle replacement compiled fine, Fedora 31's
        libtextstyle-devel from gettext 0.20.1 lacks the hyperlink support
        added upstream in gettext 0.21, causing:
        
        pk-term.c: In function 'pk_term_init':
        pk-term.c:75:21: error: lvalue required as left operand of assignment
            75 |     style_file_name = NULL;
    
    Hmm, even if the libtextstyle-devel library found in the system doesn't
    support hyperlinks, I would expect for style_file_name to not be
    replaced by gnulib.

This is the relevant macro in m4/libtextstyle-optional.m4:

AC_DEFUN([gl_LIBTEXTSTYLE_OPTIONAL],
[
  AC_REQUIRE([gl_LIBTEXTSTYLE])
  if test $HAVE_LIBTEXTSTYLE = yes; then
    TEXTSTYLE_H=
  else
    TEXTSTYLE_H=textstyle.h
    AC_REQUIRE([AC_C_INLINE])
    AC_CHECK_FUNCS_ONCE([tcdrain])
  fi
  AC_SUBST([TEXTSTYLE_H])
  AM_CONDITIONAL([GL_GENERATE_TEXTSTYLE_H], [test -n "$TEXTSTYLE_H"])
])

And in turn:

AC_DEFUN([gl_LIBTEXTSTYLE],
[
  AC_LIB_HAVE_LINKFLAGS([textstyle], [],
    [#include <textstyle.h>], 
[term_styled_ostream_create(1,"",TTYCTL_AUTO,"");],
    [no])
])

Note how gl_LIBTEXTSTYLE doesn't depend on the availability of
hyperlinks.  It is the textstyle.h provided by gl_LIBTEXTSTYLE_OPTIONAL
that #defines style_file_name to NULL...