Two important changes to the Windows build:

1) cvtres is no longer used directly. MSDN says to send the .res files to link.exe; that runs cvtres for us (at least it seems to, accoring to Google results for error LNK1158). It'll also avoid some of the weird warnings in cvtres, like CVT4001.

2) (and more important, but it depended on 1) Command-line switches are now passed using - instead of /. This is because some versions of MinGW are buggy and treat anything that starts with a / as a MSYS filename that needs to be converted to a Windows pathname.

Update #16.

GNUmakefile

@@ -31,10 +31,10 @@ endif
 
 # parameters
 export OS
-# TODO CC, CXX, RC, CVTRES, LD
+# TODO CC, CXX, RC, LD
 export CFLAGS
 export CXXFLAGS
-# TODO RCFLAGS, CVTRESFLAGS
+# TODO RCFLAGS
 export LDFLAGS
 export NODEBUG
 export EXAMPLE

build/GNUbasegcc.mk

@@ -71,8 +71,6 @@ $(OBJDIR)/%.m.o: $$(subst _,/,%).m $(HFILES) | $(OBJDIR)
 	@$(CC) -o $@ -c $< $(CFLAGS)
 	@echo ====== Compiled $<
 
-# TODO split into $(RC) and $(CVTRES) forms
-# with binutils windres can either go straight to a .o file or in the normal two steps
 $(OBJDIR)/%.rc.o: $$(subst _,/,%).rc $(HFILES) | $(OBJDIR)
 	@$(RC) $(RCFLAGS) $< $@
 	@echo ====== Compiled $<

build/GNUbasemsvc.mk

@@ -1,5 +1,19 @@
 # 16 october 2015
 
+# IMPORTANT
+# Do NOT use / for command-line options here!
+# This breaks on GNU makes that come with some versions of
+# MinGW because they mangle things that start with /, thinking that
+# those arguments are Unix paths that need to be converted to
+# Windows paths. This cannot be turned off. -_-'
+# MSDN says cl, rc, and link all accept - instead of /, so we're good.
+# See also:
+# - https://github.com/andlabs/libui/issues/16
+# - http://www.mingw.org/wiki/Posix_path_conversion
+# - http://www.mingw.org/wiki/FAQ
+# - http://stackoverflow.com/questions/7250130/how-to-stop-mingw-and-msys-from-mangling-path-names-given-at-the-command-line
+# - http://stackoverflow.com/questions/28533664/how-to-prevent-msys-to-convert-the-file-path-for-an-external-program
+
 # TODO subsystem version
 
 # TODO silence compiler non-diagnostics (/nologo is not enough)
@@ -13,27 +27,27 @@
 # TODO /analyze requires us to write annotations everywhere
 # TODO undecided flags from qo?
 CFLAGS += \
-	/W4 \
+	-W4 \
-	/wd4100 \
+	-wd4100 \
-	/TC \
+	-TC \
-	/bigobj /nologo \
+	-bigobj -nologo \
-	/RTC1 /RTCc /RTCs /RTCu
+	-RTC1 -RTCc -RTCs -RTCu
 
 CXXFLAGS += \
-	/W4 \
+	-W4 \
-	/wd4100 \
+	-wd4100 \
-	/TP \
+	-TP \
-	/bigobj /nologo \
+	-bigobj -nologo \
-	/RTC1 /RTCc /RTCs /RTCu
+	-RTC1 -RTCc -RTCs -RTCu
 
 # TODO warnings on undefined symbols
 LDFLAGS += \
-	/largeaddressaware /nologo /incremental:no
+	-largeaddressaware -nologo -incremental:no
 
 ifneq ($(NODEBUG),1)
-	CFLAGS += /Zi
+	CFLAGS += -Zi
-	CXXFLAGS += /Zi
+	CXXFLAGS += -Zi
-	LDFLAGS += /debug
+	LDFLAGS += -debug
 endif
 
 # Build rules.
@@ -48,10 +62,10 @@ OFILES := $(OFILES:%=$(OBJDIR)/%.o)
 
 OUT = $(OUTDIR)/$(NAME)$(SUFFIX)
 
-# TODO use $(CC), $(CXX), $(LD), $(RC), and $(CVTRES)
+# TODO use $(CC), $(CXX), $(LD), and s$(RC)
 
 $(OUT): $(OFILES) | $(OUTDIR)
-	@link /out:$(OUT) $(OFILES) $(LDFLAGS)
+	@link -out:$(OUT) $(OFILES) $(LDFLAGS)
 	@echo ====== Linked $(OUT)
 
 .SECONDEXPANSION:
@@ -59,23 +73,23 @@ $(OUT): $(OFILES) | $(OUTDIR)
 # TODO can we put /Fd$@.pdb in a variable?
 $(OBJDIR)/%.c.o: $$(subst _,/,%).c $(HFILES) | $(OBJDIR)
 ifeq ($(NODEBUG),1)
-	@cl /Fo:$@ /c $< $(CFLAGS)
+	@cl -Fo:$@ -c $< $(CFLAGS)
 else
-	@cl /Fo:$@ /c $< $(CFLAGS) /Fd$@.pdb
+	@cl -Fo:$@ -c $< $(CFLAGS) -Fd$@.pdb
 endif
 	@echo ====== Compiled $<
 
 $(OBJDIR)/%.cpp.o: $$(subst _,/,%).cpp $(HFILES) | $(OBJDIR)
 ifeq ($(NODEBUG),1)
-	@cl /Fo:$@ /c $< $(CXXFLAGS)
+	@cl -Fo:$@ -c $< $(CXXFLAGS)
 else
-	@cl /Fo:$@ /c $< $(CXXFLAGS) /Fd$@.pdb
+	@cl -Fo:$@ -c $< $(CXXFLAGS) -Fd$@.pdb
 endif
 	@echo ====== Compiled $<
 
+# note: don't run cvtres directly; the linker does that for us
 $(OBJDIR)/%.rc.o: $$(subst _,/,%).rc $(HFILES) | $(OBJDIR)
-	@rc /nologo /v /fo $@.res $<
+	@rc -nologo -v -fo $@ $<
-	@cvtres /nologo /out:$@ $@.res
 	@echo ====== Compiled $<
 
 $(OBJDIR) $(OUTDIR):

windows/GNUfiles.mk

@@ -62,7 +62,7 @@ LDFLAGS += \
 
 # flags for building a shared library
 LDFLAGS += \
-	/dll
+	-dll
 
 # TODO flags for warning on undefined symbols