[tlbuild] Building with -C fails to find libpng

[Mojca Miklavec]

Hi,

After discussing with Norbert, he mentioned that one could
significantly speed up the build by caching the configure results with
"-C".

But for some super weird reason that broke my build for i386-darwin &
powerpc-darwin. Other builds work just fine.

Mojca


=== configuring in gd (/path/to/build/Work/libs/gd)
...

checking build system type... (cached) i386-apple-darwin10.8.0
checking host system type... (cached) i386-apple-darwin10.8.0
checking for pkg-config... (cached) pkg-config
checking for deflate... yes
checking zlib.h usability... yes
checking zlib.h presence... yes
checking for zlib.h... yes
checking for png_create_read_struct... no
configure: error: libpng not found
=== configuring in gd failed
make[2]: *** [recurse] Error 1
make[1]: *** [all-recursive] Error 1
make: *** [all-recursive] Error 1
+ echo 2