Calculate SO_VERSION in configure for compatibility with BSD make.

BSD Make does not support `$(shell ...)`. It does support an alternative, `!=` assignments (which, confusingly, don't mean "not equal" but rather "evaluate the right in the shell before assignment"). GNU Make also supports `!=` as of version 4.0, released in 2013. Unfortunately, f***ing Apple ships GNU Make version 3.81, from 2006, with MacOS/XCode.

c++/Makefile.am

@@ -235,10 +235,6 @@ else
 lib_LTLIBRARIES = libkj.la libkj-test.la libkj-async.la libkj-http.la $(MAYBE_KJ_TLS_LA) libcapnp.la libcapnp-rpc.la libcapnp-json.la libcapnpc.la
 endif
 
-# Don't include security release in soname -- we want to replace old binaries
-# in this case.
-SO_VERSION = $(shell echo $(VERSION) | sed -e 's/^\([0-9]*[.][0-9]*[.][0-9]*\)\([.][0-9]*\)*\(-.*\)*$$/\1\3/g')
-
 libkj_la_LIBADD = $(PTHREAD_LIBS)
 libkj_la_LDFLAGS = -release $(SO_VERSION) -no-undefined
 libkj_la_SOURCES=                                              \

c++/configure.ac

@@ -129,6 +129,11 @@ AC_DEFUN([CAPNP_CMAKE_CONFIG_FILES], [ \
 AC_SUBST([CAPNP_PKG_CONFIG_FILES])
 AC_SUBST([CAPNP_CMAKE_CONFIG_FILES])
 
+# Don't include security release in soname -- we want to replace old binaries
+# in this case.
+SO_VERSION=$(echo $VERSION | sed -e 's/^\([0-9]*[.][0-9]*[.][0-9]*\)\([.][0-9]*\)*\(-.*\)*$/\1\3/g')
+AC_SUBST([SO_VERSION])
+
 # CapnProtoConfig.cmake.in needs these PACKAGE_* output variables.
 PACKAGE_INIT="set([CAPNP_PKG_CONFIG_FILES] CAPNP_PKG_CONFIG_FILES)"
 PACKAGE_CMAKE_INSTALL_FULL_INCLUDEDIR="\${CMAKE_CURRENT_LIST_DIR}/../../../include"