builld: allow out of source build * Makefile.in: add TOP_BUILD_DIR var and use TOP_DIR=srcdir, TOP_BUILD_DIR=builddir, modify paths to use TOP_DIR where needed * binary-dist-rules.mk: use TOP_DIR and TOP_BUILD_DIR where needed * makeinst-script.sh: use script path to determine TOPDIR, use TOPDIR where needed * src/default-octave.mk: install octave-version to builddir * src/stable-octave.mk: install octave-version to builddir * src/octave.mk: install octave-version to builddir * tools/set-mxe-env.sh.in: update fir builddir and srcdir use

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Make sure exec_prefix is defined in wavpack.pc.

diff -ur a/wavpack.pc.in b/wavpack.pc.in
--- a/wavpack.pc.in	2013-09-29 23:30:14.000000000 +0200
+++ b/wavpack.pc.in	2013-11-17 18:31:35.351567596 +0100
@@ -1,4 +1,5 @@
 prefix=@prefix@
+exec_prefix=${prefix}
 libdir=@libdir@
 includedir=${prefix}/include