Problem: source packages lack makefiles for all systems except msvc

Solution: specify the necessary EXTRA_DIST

I added a Makefile.am in builds that covers all systems except msvc,
which already has a Makefile.am that does this.

Fixes #1505

Makefile.am

@@ -2,7 +2,7 @@ ACLOCAL_AMFLAGS = -I config
 
 SUBDIRS = doc
 
-DIST_SUBDIRS = builds/msvc doc
+DIST_SUBDIRS = doc builds builds/msvc
 
 pkgconfig_DATA = src/libzmq.pc
 

builds/Makefile.am

@@ -0,0 +1,26 @@
+#  Specify all build files that have to go into source packages.
+#  msvc directory does its own stuff.
+
+EXTRA_DIST = \
+	zos/makelibzmq \
+	zos/cxxall \
+	zos/README.md \
+	zos/makeclean \
+	zos/platform.hpp \
+	zos/zc++ \
+	zos/test_fork.cpp \
+	zos/maketests \
+	zos/runtests \
+	mingw32/Makefile.mingw32 \
+	mingw32/platform.hpp \
+	cmake/Modules \
+	cmake/Modules/FindAsciiDoc.cmake \
+	cmake/Modules/TestZMQVersion.cmake \
+	cmake/Modules/ZMQSourceRunChecks.cmake \
+	cmake/NSIS.template32.in \
+	cmake/platform.hpp.in \
+	cmake/NSIS.template64.in \
+	valgrind/valgrind.supp \
+	valgrind/vg \
+	nuget/readme.nuget \
+	nuget/libzmq.autopkg

configure.ac

@@ -560,6 +560,7 @@ AC_CONFIG_FILES([ \
     Makefile \
     src/libzmq.pc \
     doc/Makefile \
+    builds/Makefile \
     builds/msvc/Makefile \
     builds/redhat/zeromq.spec])