Fix libfftw3/libfftw3_threads chicken-egg problem

On most systems we want to build libfftw3 first, so that
libfftw3_threads can depend upon libfftw3.  When producing a single
combined-thread library (e.g. on Windows) we want the opposite,
so that libfftw3 can include libfftw3_threads.

Makefile.am

@@ -13,8 +13,25 @@ endif
 
 ACLOCAL_AMFLAGS=-I m4
 
+# when using combined thread libraries (necessary on Windows), we want
+# to build threads/ first, because libfftw3_threads is added to
+# libfftw3.
+#
+# Otherwise, we want to build libfftw3_threads after libfftw3
+# so that we can track the fact that libfftw3_threads depends upon
+# libfftw3.
+#
+# This is the inescapable result of combining three bad ideas
+# (threads, Windows, and shared libraries).
+#
+if COMBINED_THREADS
+CHICKEN_EGG=threads .
+else
+CHICKEN_EGG=. threads
+endif
+
 SUBDIRS=support $(GENFFT) kernel simd-support dft rdft reodft api	\
-libbench2 . threads tests mpi doc tools m4
+libbench2 $(CHICKEN_EGG) tests mpi doc tools m4
 EXTRA_DIST=COPYRIGHT bootstrap.sh CONVENTIONS fftw.pc.in
 
 SIMD_LIBS =						\

threads/Makefile.am

@@ -26,10 +26,15 @@ threads.h dft-vrank-geq1.c ct.c rdft-vrank-geq1.c hc2hc.c		\
 vrank-geq1-rdft2.c f77api.c f77funcs.h
 libfftw3@PREC_SUFFIX@_threads_la_CFLAGS = $(AM_CFLAGS) $(PTHREAD_CFLAGS)
 libfftw3@PREC_SUFFIX@_threads_la_LDFLAGS = -version-info @SHARED_VERSION_INFO@
+if !COMBINED_THREADS
 libfftw3@PREC_SUFFIX@_threads_la_LIBADD = ../libfftw3@[email protected]
+endif
 
 libfftw3@PREC_SUFFIX@_omp_la_SOURCES = api.c conf.c openmp.c	\
 threads.h dft-vrank-geq1.c ct.c rdft-vrank-geq1.c hc2hc.c	\
 vrank-geq1-rdft2.c f77api.c f77funcs.h
 libfftw3@PREC_SUFFIX@_omp_la_CFLAGS = $(AM_CFLAGS) $(OPENMP_CFLAGS)
 libfftw3@PREC_SUFFIX@_omp_la_LDFLAGS = -version-info @SHARED_VERSION_INFO@
+if !COMBINED_THREADS
+libfftw3@PREC_SUFFIX@_omp_la_LIBADD = ../libfftw3@[email protected]
+endif