allow for build directory different than source directory

[sthalik]

The autotools machinery assumes top_srcdir to be the root of the build directory. This breaks building in different locations, like "build/" subdirectory.

[tpoechtrager]

Thanks! Please add this minor change to your PR to avoid breaking the LTO build:

diff --git a/cctools/m4/llvm.m4 b/cctools/m4/llvm.m4
index a3e28ec..29dafca 100644
--- a/cctools/m4/llvm.m4
+++ b/cctools/m4/llvm.m4
@@ -39,7 +39,7 @@ AC_DEFUN([CHECK_LLVM],
                LTO_DEF=-DLTO_SUPPORT
                # DO NOT include the LLVM include dir directly,
                # it may cause the build to fail.
-               `cp -r $LLVM_INCLUDE_DIR/llvm-c/lto.h include/llvm-c/lto.h 2>/dev/null 1>&2`
+               cp -f $LLVM_INCLUDE_DIR/llvm-c/lto.h `dirname ${0}`/include/llvm-c/lto.h
                AC_SUBST([LTO_DEF])
                AC_SUBST([LTO_RPATH])
                AC_SUBST([LTO_LIB]) ])

[sthalik]

Rebased.