sagemathgh-39063: PKG_CONFIG must be empty if not found

Followup to sagemath#38954, where I
followed the pkgconf documentation to set `PKG_CONFIG=false` if it is
not found. But Sage uses `"test -z $PKG_CONFIG"`, so it must instead be
set to empty.

This causes pkgconf not to be built if pkgconf/pkg-config is not found.
Which is basically only macOS.

URL: sagemath#39063
Reported by: Volker Braun
Reviewer(s):

build/pkgs/configure/checksums.ini

@@ -1,3 +1,3 @@
 tarball=configure-VERSION.tar.gz
-sha1=4d6de7b4a5aff43a508a1e64078fe409554869fe
-sha256=18cc6fa95c0bd6ae0bf1f9b447ae2dd9da7d596d17bec5e961b980b9687e1a7e
+sha1=7d0d584233b0a52b62dba5eaeaac13d8c38652eb
+sha256=02d5e5ee0fad7ccbc99ae92ecb7cfc92260962cb1c8714c6fcd2f561c3d718e6

build/pkgs/configure/package-version.txt

@@ -1 +1 @@
-68f446ad3f1abcea6e1a9b679a04242597f439f6
+a60f0455273de4b2b07878d92e8357d8d9a839c8

configure.ac

@@ -222,7 +222,7 @@ dnl Exit autoconf with exit code 16 in this case. This will be
 dnl caught by the bootstrap script.
 m4_exit(16)])
 
-PKG_PROG_PKG_CONFIG([0.29], [PKG_CONFIG=false])
+PKG_PROG_PKG_CONFIG([0.29], [PKG_CONFIG=])
 
 AC_CHECK_PROG(found_ranlib, ranlib, yes, no)
 if test x$found_ranlib != xyes