As reported downstream:
https://bugs.gentoo.org/show_bug.cgi?id=493254

libSM looks to also be required as, otherwise, configure tells Xtst is not found even when it's present:
configure:13408: checking for XTestFakeKeyEvent in -lXtst
configure:13433: x86_64-pc-linux-gnu-gcc -o conftest -O2 -march=x86-64 -pipe   -Wl,-O1 -Wl,--as-needed conftest.c -lXtst   -lSM -lICE -lX11   >&5
/usr/lib/gcc/x86_64-pc-linux-gnu/4.7.3/../../../../x86_64-pc-linux-gnu/bin/ld: cannot find -lSM
collect2: error: ld returned 1 exit status
configure:13433: $? = 1
configure: failed program was:
| /* confdefs.h */
| #define PACKAGE_NAME "at-spi2-core"
| #define PACKAGE_TARNAME "at-spi2-core"
| #define PACKAGE_VERSION "2.8.0"
| #define PACKAGE_STRING "at-spi2-core 2.8.0"
| #define PACKAGE_BUGREPORT "accessibility-atspi@lists.linux-foundation.org"
| #define PACKAGE_URL ""
| #define PACKAGE "at-spi2-core"
| #define VERSION "2.8.0"
| #define STDC_HEADERS 1
| #define HAVE_SYS_TYPES_H 1
| #define HAVE_SYS_STAT_H 1
| #define HAVE_STDLIB_H 1
| #define HAVE_STRING_H 1
| #define HAVE_MEMORY_H 1
| #define HAVE_STRINGS_H 1
| #define HAVE_INTTYPES_H 1
| #define HAVE_STDINT_H 1
| #define HAVE_UNISTD_H 1
| #define HAVE_DLFCN_H 1
| #define LT_OBJDIR ".libs/"
| /* end confdefs.h.  */
| 
| /* Override any GCC internal prototype to avoid an error.
|    Use char because int might match the return type of a GCC
|    builtin and then its argument prototype would still apply.  */
| #ifdef __cplusplus
| extern "C"
| #endif
| char XTestFakeKeyEvent ();
| int
| main ()
| {
| return XTestFakeKeyEvent ();
|   ;
|   return 0;
| }
configure:13442: result: no
configure:13448: error: Couldn't find the Xtst library. Check config.log

Is this normal?

Thanks