Build Windows DLLs with `-Wl,--export-all-symbols`

This is not the most elegant, but will match the SOs in exporting
everything for now. Later we can refine what is public/private to clean
up the interface.

Makefile

@@ -1,8 +1,12 @@
+# External build directory support
+
 include mk/build-dir.mk
 
 -include $(buildprefix)Makefile.config
 clean-files += $(buildprefix)Makefile.config
 
+# List makefiles
+
 ifeq ($(ENABLE_BUILD), yes)
 makefiles = \
   mk/precompiled-headers.mk \
@@ -43,6 +47,8 @@ makefiles += \
   tests/functional/plugins/local.mk
 endif
 
+# Miscellaneous global Flags
+
 OPTIMIZE = 1
 
 ifeq ($(OPTIMIZE), 1)
@@ -52,9 +58,29 @@ else
   GLOBAL_CXXFLAGS += -O0 -U_FORTIFY_SOURCE
 endif
 
+include mk/platform.mk
+
+ifdef HOST_WINDOWS
+  # Windows DLLs are stricter about symbol visibility than Unix shared
+  # objects --- see https://gcc.gnu.org/wiki/Visibility for details.
+  # This is a temporary sledgehammer to export everything like on Unix,
+  # and not detail with this yet.
+  #
+  # TODO do not do this, and instead do fine-grained export annotations.
+  GLOBAL_LDFLAGS += -Wl,--export-all-symbols
+endif
+
+GLOBAL_CXXFLAGS += -g -Wall -include $(buildprefix)config.h -std=c++2a -I src
+
+# Include the main lib, causing rules to be defined
+
 include mk/lib.mk
 
-# Must be included after `mk/lib.mk` so isn't the default target.
+# Fallback stub rules for better UX when things are disabled
+#
+# These must be defined after `mk/lib.mk`. Otherwise the first rule
+# incorrectly becomes the default target.
+
 ifneq ($(ENABLE_UNIT_TESTS), yes)
 .PHONY: check
 check:
@@ -69,8 +95,11 @@ installcheck:
 	@exit 1
 endif
 
-# Must be included after `mk/lib.mk` so rules refer to variables defined
-# by the library. Rules are not "lazy" like variables, unfortunately.
+# Documentation or else fallback stub rules.
+#
+# The documentation makefiles be included after `mk/lib.mk` so rules
+# refer to variables defined by `mk/lib.mk`. Rules are not "lazy" like
+# variables, unfortunately.
 
 ifeq ($(ENABLE_DOC_GEN), yes)
 $(eval $(call include-sub-makefile, doc/manual/local.mk))
@@ -89,5 +118,3 @@ internal-api-html:
 	@echo "Internal API docs are disabled. Configure with '--enable-internal-api-docs', or avoid calling 'make internal-api-html'."
 	@exit 1
 endif
-
-GLOBAL_CXXFLAGS += -g -Wall -include $(buildprefix)config.h -std=c++2a -I src

mk/lib.mk

@@ -12,38 +12,7 @@ man-pages :=
 install-tests :=
 install-tests-groups :=
 
-ifdef HOST_OS
-  HOST_KERNEL = $(firstword $(subst -, ,$(HOST_OS)))
-  ifeq ($(patsubst mingw%,,$(HOST_KERNEL)),)
-    HOST_MINGW = 1
-    HOST_WINDOWS = 1
-  endif
-  ifeq ($(HOST_KERNEL), cygwin)
-    HOST_CYGWIN = 1
-    HOST_WINDOWS = 1
-    HOST_UNIX = 1
-  endif
-  ifeq ($(patsubst darwin%,,$(HOST_KERNEL)),)
-    HOST_DARWIN = 1
-    HOST_UNIX = 1
-  endif
-  ifeq ($(patsubst freebsd%,,$(HOST_KERNEL)),)
-    HOST_FREEBSD = 1
-    HOST_UNIX = 1
-  endif
-  ifeq ($(patsubst netbsd%,,$(HOST_KERNEL)),)
-    HOST_NETBSD = 1
-    HOST_UNIX = 1
-  endif
-  ifeq ($(HOST_KERNEL), linux)
-    HOST_LINUX = 1
-    HOST_UNIX = 1
-  endif
-  ifeq ($(patsubst solaris%,,$(HOST_KERNEL)),)
-    HOST_SOLARIS = 1
-    HOST_UNIX = 1
-  endif
-endif
+include mk/platform.mk
 
 # Hack to define a literal space.
 space :=

mk/platform.mk

@@ -0,0 +1,32 @@
+ifdef HOST_OS
+  HOST_KERNEL = $(firstword $(subst -, ,$(HOST_OS)))
+  ifeq ($(patsubst mingw%,,$(HOST_KERNEL)),)
+    HOST_MINGW = 1
+    HOST_WINDOWS = 1
+  endif
+  ifeq ($(HOST_KERNEL), cygwin)
+    HOST_CYGWIN = 1
+    HOST_WINDOWS = 1
+    HOST_UNIX = 1
+  endif
+  ifeq ($(patsubst darwin%,,$(HOST_KERNEL)),)
+    HOST_DARWIN = 1
+    HOST_UNIX = 1
+  endif
+  ifeq ($(patsubst freebsd%,,$(HOST_KERNEL)),)
+    HOST_FREEBSD = 1
+    HOST_UNIX = 1
+  endif
+  ifeq ($(patsubst netbsd%,,$(HOST_KERNEL)),)
+    HOST_NETBSD = 1
+    HOST_UNIX = 1
+  endif
+  ifeq ($(HOST_KERNEL), linux)
+    HOST_LINUX = 1
+    HOST_UNIX = 1
+  endif
+  ifeq ($(patsubst solaris%,,$(HOST_KERNEL)),)
+    HOST_SOLARIS = 1
+    HOST_UNIX = 1
+  endif
+endif