Blame m4/ax_require_defined.m4

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# ===========================================================================
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#    https://www.gnu.org/software/autoconf-archive/ax_require_defined.html
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# ===========================================================================
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#
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# SYNOPSIS
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#
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#   AX_REQUIRE_DEFINED(MACRO)
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#
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# DESCRIPTION
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#
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#   AX_REQUIRE_DEFINED is a simple helper for making sure other macros have
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#   been defined and thus are available for use.  This avoids random issues
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#   where a macro isn't expanded.  Instead the configure script emits a
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#   non-fatal:
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#
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#     ./configure: line 1673: AX_CFLAGS_WARN_ALL: command not found
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#
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#   It's like AC_REQUIRE except it doesn't expand the required macro.
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#
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#   Here's an example:
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#
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#     AX_REQUIRE_DEFINED([AX_CHECK_LINK_FLAG])
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#
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# LICENSE
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#
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#   Copyright (c) 2014 Mike Frysinger <vapier@gentoo.org>
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#
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#   Copying and distribution of this file, with or without modification, are
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#   permitted in any medium without royalty provided the copyright notice
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#   and this notice are preserved. This file is offered as-is, without any
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#   warranty.
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#serial 2
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AC_DEFUN([AX_REQUIRE_DEFINED], [dnl
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  m4_ifndef([$1], [m4_fatal([macro ]$1[ is not defined; is a m4 file missing?])])
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])dnl AX_REQUIRE_DEFINED