Makefile.m32: add support for UNICODE builds

It requires the linker to support the `-municode` option.
This is available in more recent mingw-w64 releases.

Ref: https://gcc.gnu.org/onlinedocs/gcc/x86-Windows-Options.html
Ref: https://stackoverflow.com/questions/3571250/wwinmain-unicode-and-mingw/11706847#11706847

Reviewed-by: Jay Satiro
Reviewed-by: Marcel Raad

Closes #6228

lib/Makefile.m32

@@ -226,6 +226,9 @@ endif
 ifeq ($(findstring -ngtcp2,$(CFG)),-ngtcp2)
 NGTCP2 = 1
 endif
+ifeq ($(findstring -unicode,$(CFG)),-unicode)
+UNICODE = 1
+endif
 
 INCLUDES = -I. -I../include
 CFLAGS += -DBUILDING_LIBCURL
@@ -234,6 +237,9 @@ ifdef SSL
     CFLAGS += -DCURL_WITH_MULTI_SSL
   endif
 endif
+ifdef UNICODE
+  CFLAGS += -DUNICODE -D_UNICODE
+endif
 
 ifdef SYNC
   CFLAGS += -DUSE_SYNC_DNS

src/Makefile.m32

@@ -238,6 +238,9 @@ endif
 ifeq ($(findstring -ngtcp2,$(CFG)),-ngtcp2)
 NGTCP2 = 1
 endif
+ifeq ($(findstring -unicode,$(CFG)),-unicode)
+UNICODE = 1
+endif
 
 INCLUDES = -I. -I../include -I../lib
 ifdef SSL
@@ -245,6 +248,10 @@ ifdef SSL
     CFLAGS += -DCURL_WITH_MULTI_SSL
   endif
 endif
+ifdef UNICODE
+  CFLAGS += -DUNICODE -D_UNICODE
+  LDFLAGS += -municode
+endif
 
 ifdef DYN
   curl_DEPENDENCIES = $(PROOT)/lib/libcurldll.a $(PROOT)/lib/libcurl.dll