[PATCH] autoconf: fix build on upcoming GNU make-4.4

[Sergei Trofimovich]

>From https://savannah.gnu.org/bugs/?63040:
GNU make 4.4 will slightly change the rules it uses to export variables
to $(shell ...) calls. `autoconf` uses unportable variant of assignment:

    export PATH = $(shell echo "`pwd`/tests:$$PATH")

This causes build failure on `make` from `git`:

    bash: line 1: env: command not found
    make  all-am
    bash: line 1: make: command not found
    make: *** [Makefile:928: all] Error 127

The change it to use probably intended for or eager assignment:

    export PATH = $(shell echo "`pwd`/tests:$$PATH")

        * cfg.mk (PATH): Assign path after $(shell) call.
---
 cfg.mk | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/cfg.mk b/cfg.mk
index 7e515130..fd34dfa0 100644
--- a/cfg.mk
+++ b/cfg.mk
@@ -18,7 +18,7 @@
 # This file is '-include'd into GNUmakefile.
 
 # Build with our own versions of these tools, when possible.
-export PATH = $(shell echo "`pwd`/tests:$$PATH")
+export PATH := $(shell echo "`pwd`/tests:$$PATH")
 
 # Remove the autoreconf-provided INSTALL, so that we regenerate it.
 _autoreconf = autoreconf -i -v && rm -f INSTALL
-- 
2.37.2