bug#18779: 24.4; Compilation error on Mac OS X Yosemite

[Constantine Vetoshev]

I think something changed between 10.9 (Mavericks) and 10.10
(Yosemite) with include file locations.

After running "./configure --with-ns && make install", the build proceeds until:

gcc -c  -Demacs  -I. -I. -I../lib -I./../lib
-I/usr/include/libxml2          -MMD -MF deps/xml.d -MP   -Wno-switch
-Wno-tautological-constant-out-of-range-compare -Wno-pointer-sign  -g3
-O2  xml.c
xml.c:23:10: fatal error: 'libxml/tree.h' file not found
#include <libxml/tree.h>
         ^
1 error generated.
make[1]: *** [xml.o] Error 1
make: *** [src] Error 2

With the latest Xcode 6.1, symlinking
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.10.sdk/usr/include
to /usr/include makes the compilation error go away. The compiled
binary seems to work.