build-pkg: add usage notes for pkg list and dry run

Makefile

@@ -705,7 +705,7 @@ $(PREFIX)/$(3)/installed/$(1): $(PKG_MAKEFILES) \
 	    | $(RTRIM)
 	)
 	$(if $(value $(call LOOKUP_PKG_RULE,$(1),BUILD,$(3))),
-	    $(if $(BUILD_DRY_RUN), \
+	    $(if $(BUILD_DRY_RUN)$(MXE_BUILD_DRY_RUN), \
 	        @$(PRINTF_FMT) '[dry-run]' '$(1)' '$(3)' | $(RTRIM)
 	        @[ -d '$(PREFIX)/$(3)/lib' ] || mkdir -p '$(PREFIX)/$(3)/lib'
 	        @touch '$(PREFIX)/$(3)/lib/$(1).dry-run'

tools/build-pkg.lua

@@ -20,6 +20,10 @@ To prevent build-pkg from creating deb packages,
 set environment variable MXE_BUILD_PKG_NO_DEBS to 1
 In this case fakeroot and dpkg-deb are not needed.
 
+To do a dry run without actually building any packages,
+set environment variable MXE_BUILD_DRY_RUN to any value
+Packages will be downloaded, but builds will be skipped.
+
 To switch off the second pass, set
 MXE_BUILD_PKG_NO_SECOND_PASS to 1.
 See https://github.com/mxe/mxe/issues/1111
@@ -32,6 +36,13 @@ set environment variable MXE_BUILD_PKG_TARGETS to
 the list of targets separated by space.
 By default, all 4 major targets are built.
 
+To set list of MXE packages to build,
+set environment variable MXE_BUILD_PKG_PKGS to
+the list of packages separated by space. This is similar
+to a normal `make` invocation in that all dependencies
+will be built, so list just the packages you require.
+By default, all packages are built.
+
 The following error:
 > fakeroot, while creating message channels: Invalid argument
 > This may be due to a lack of SYSV IPC support.
@@ -48,7 +59,9 @@ make -C $MXE_DIR lua \
     lua_TARGETS=$BUILD \
     PREFIX=$MXE_DIR/usr.lua && \
 MXE_BUILD_PKG_TARGETS="`echo {i686-w64-mingw32,x86_64-w64-mingw32}.{static,shared}`" \
-MXE_BUILD_PKG_MAX_ITEMS=10000 \
+MXE_BUILD_PKG_PKGS= \
+MXE_BUILD_DRY_RUN=1 \
+MXE_BUILD_PKG_MAX_ITEMS= \
 MXE_BUILD_PKG_NO_DEBS=1 \
 MXE_BUILD_PKG_NO_SECOND_PASS=0 \
 $MXE_DIR/usr.lua/$BUILD/bin/lua $MXE_DIR/tools/build-pkg.lua