This is a follow-up to r291561 which reworked the bootstrap tool PATH
handling.
An example of this is when building lib/clang/libclangedit. At first
clang-tblgen will not be staged in the host tree so it will have
CLANG_TBLGEN=clang-tblgen set and exported. During the build though it
will stage clang-tblgen and then find it via the PATH. On the next
build it finds clang-tblgen in the stage directory and would set
CLANG_TBLGEN=<stagedir/usr/bin>/clang-tblgen thus causing the build
command to change. In both cases the same exact tool was used though so
there is no need to rebuild. If the tool did change the normal
meta/filemon handling would pick that up via timestamp comparisons and
rebuild.
Sponsored by: EMC / Isilon Storage Division
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