Centos7 strongswan configure.ac PKG_CHECK_VAR

在configure.ac前邊添加以下內容：

# backwards compat with older pkg-config
# - pull in AC_DEFUN from pkg.m4
m4_ifndef([PKG_CHECK_VAR], [
# PKG_CHECK_VAR(VARIABLE, MODULE, CONFIG-VARIABLE,
# [ACTION-IF-FOUND], [ACTION-IF-NOT-FOUND])
# -------------------------------------------
# Retrieves the value of the pkg-config variable for the given module.
AC_DEFUN([PKG_CHECK_VAR],
[AC_REQUIRE([PKG_PROG_PKG_CONFIG])dnl
AC_ARG_VAR([$1], [value of $3 for $2, overriding pkg-config])dnl
_PKG_CONFIG([$1], [variable="][$3]["], [$2])
AS_VAR_COPY([$1], [pkg_cv_][$1])
AS_VAR_IF([$1], [""], [$5], [$4])dnl
])# PKG_CHECK_VAR
])

參考：https://blog.csdn.net/a363344923/article/details/105417015