The function strstr() in compat/strstr.c uses NULL as a
return value but NULL is not defined, neither locally nor
by inclusion of an appropriate header file.
I discovered this when cross-compiling the latest
tcl8.4.9 source code on a Linux x86 platform targetting
an embedded PowerPC 60x running Linux. The
configure script correctly detected that the strstr()
function needed to be build for this platform, but the
subsequent compilation failed because of the undefined
NULL. Fixing is trivial.