NMake Makefiles: Support Meson-built libsigc++

Add an option USE_MESON_LIBS for the NMake command line so that we can use
the Meson-built libsigc++ easier, without needing to manually update Makefiles

MSVC_NMake/config-msvc.mak

@@ -50,9 +50,14 @@ LIBPANGOMM_CFLAGS = /DPANGOMM_BUILD $(PANGOMM_BASE_CFLAGS) $(PANGOMM_EXTRA_INCLU
 # We build pangomm-vc$(VSVER)0-$(PANGOMM_MAJOR_VERSION)_$(PANGOMM_MINOR_VERSION).dll or
 #          pangomm-vc$(VSVER)0-d-$(PANGOMM_MAJOR_VERSION)_$(PANGOMM_MINOR_VERSION).dll at least
 
+!ifdef USE_MESON_LIBS
+LIBSIGC_LIBNAME = sigc-$(LIBSIGC_MAJOR_VERSION).$(LIBSIGC_MINOR_VERSION)
+LIBSIGC_DLL = $(LIBSIGC_LIBNAME)-0.dll
+!else
 LIBSIGC_LIBNAME = sigc-vc$(VSVER)0$(DEBUG_SUFFIX)-$(LIBSIGC_MAJOR_VERSION)_$(LIBSIGC_MINOR_VERSION)
-
 LIBSIGC_DLL = $(LIBSIGC_LIBNAME).dll
+!endif
+
 LIBSIGC_LIB = $(LIBSIGC_LIBNAME).lib
 
 GLIBMM_LIBNAME = glibmm-vc$(VSVER)0$(DEBUG_SUFFIX)-$(GLIBMM_MAJOR_VERSION)_$(GLIBMM_MINOR_VERSION)

README.win32

@@ -52,7 +52,9 @@ all the depedent libraries could be found in $(PREFIX), which is at
 $(srcroot)\..\vs15\$(Platform) by default.  Run
 'nmake /f Makefile.vc CFG=[release|debug]' to build pangomm.  If a
 different $(PREFIX) is desired rather than the aforementioned default,
-pass in PREFIX=$(PATH) into your command line.
+pass in PREFIX=$(PATH) into your command line.  If using C++
+dependencies that are built with Meson, specify USE_MESON_LIBS=1 in your
+NMake command line.
 
 A 'clean' target is supported to remove all the built object files and
 intermediate files that are generated during the build, while an