unexport PKG_CONFIG_SYSROOT_DIR

If this is not done then pkg-config can get confused.

Thomas and Arnout really deserve the credit for this - I just did
the testing.

Signed-off-by: Charles Manning <cdhmanning@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

Makefile

@@ -172,6 +172,7 @@ export HOSTCC_NOCCACHE HOSTCXX_NOCCACHE
 
 # Make sure pkg-config doesn't look outside the buildroot tree
 unexport PKG_CONFIG_PATH
+unexport PKG_CONFIG_SYSROOT_DIR
 
 # Having DESTDIR set in the environment confuses the installation
 # steps of some packages.