[PATCH] configure: fix reference to possibly undefined $PKG_CONFIG_PATH

[Tomi Ollila]

In case zlib not found by pkg-config(1) the pkg-config information
is resolved by attempting to print ZLIB_VERSION from from zlib
installation if it exists anyway.

If above done successfully compat/zlib.pc is written for forthcoming
pkg-config execution.

Since `set -u` is in effect (since 124a67e96, 2016-05-06),
expanding unset $PKG_CONFIG_PATH (would have) failed whenever tried.

Now it is changed to set as "$PKG_CONFIG_PATH:compat" if PKG_CONFIG_PATH
is set and is non-empty string, plain "compat" otherwise.
---
 configure | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/configure b/configure
index 2160be37..00a9d03d 100755
--- a/configure
+++ b/configure
@@ -606,7 +606,7 @@ fi
 if ! pkg-config --exists zlib; then
   ${CC} -o compat/gen_zlib_pc "$srcdir"/compat/gen_zlib_pc.c >/dev/null 2>&1 &&
   compat/gen_zlib_pc > compat/zlib.pc &&
-  PKG_CONFIG_PATH="$PKG_CONFIG_PATH":compat &&
+  PKG_CONFIG_PATH=${PKG_CONFIG_PATH:+$PKG_CONFIG_PATH:}compat &&
   export PKG_CONFIG_PATH
   rm -f compat/gen_zlib_pc
 fi
-- 
2.23.0