If anyone is considering upgrading their development machine to OSX 10.9 ...

... then be warned that Apple, in their infinite wisdom, have decided
that people can do without "/usr/include". Of course, to make your life
more interesting, "apr-1-config" still returns "/usr/include/apr-1".

I decided on this workaround:

$ ls -l /usr
total 16
drwxr-xr-x 5 root wheel 170 Aug 25 07:05 X11
lrwxr-xr-x 1 root wheel 3 Jan 26 02:32 X11R6 -> X11
drwxr-xr-x 1083 root wheel 36822 Jan 26 04:07 bin
lrwxr-xr-x 1 root wheel 110 Jan 26 04:17 include -> /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.9.sdk/usr/include
drwxr-xr-x 263 root wheel 8942 Jan 26 02:38 lib
drwxr-xr-x 166 root wheel 5644 Jan 26 04:07 libexec
drwxr-xr-x 7 root wheel 238 Aug 5 2012 llvm-gcc-4.2
drwxrwxr-x 26 root admin 884 Jan 26 02:38 local
drwxr-xr-x 244 root wheel 8296 Jan 26 02:38 sbin
drwxr-xr-x 46 root wheel 1564 Jan 26 02:38 share
drwxr-xr-x 4 root wheel 136 Jan 26 02:25 standalone
drwxr-xr-x 3 root wheel 102 Dec 20 2012 tmp

-- Brane

-- 
Branko Čibej | Director of Subversion
WANdisco // Non-Stop Data
e. brane_at_wandisco.com