build: fix djgpp builds

- Update build instructions in packages/DOS/README

- Extend 'VPATH' with 'vquic' and 'vssh'.

- Allow 'Makefile.dist' to build both 'lib' and 'src'.

- Allow using the Windows hosted djgpp cross compiler to build for MSDOS
  under Windows.

- 'USE_SSL' -> 'USE_OPENSSL'

- Added a 'link_EXE' macro. Etc, etc.

- Linking 'curl.exe' needs '$(CURLX_CFILES)' too.

- Do not pick-up '../lib/djgpp/*.o' files. Recompile locally.

- Generate a gzipped 'tool_hugehelp.c' if 'USE_ZLIB=1'.

- Remove 'djgpp-clean'

- Adapt to new C-ares directory structure

- Use conditional variable assignments

Clarify the 'conditional variable assignment' in 'common.dj'.

Closes https://github.com/curl/curl/pull/6382

src/makefile.dj

@@ -29,13 +29,15 @@ DEPEND_PREREQ = # tool_hugehelp.c
 
 TOPDIR = ..
 
+vpath %.c ../lib
+
 include ../packages/DOS/common.dj
 include Makefile.inc
 
-CSOURCES = $(CURL_CFILES)
+CSOURCES = $(CURL_CFILES) $(CURLX_CFILES)
 
-ifeq ($(USE_SSL),1)
-  EX_LIBS += $(OPENSSL_ROOT)/lib/libssl.a $(OPENSSL_ROOT)/lib/libcrypt.a
+ifeq ($(USE_OPENSSL),1)
+  EX_LIBS += $(OPENSSL_LIBS)
 endif
 
 ifeq ($(USE_ARES),1)
@@ -53,8 +55,8 @@ endif
 
 EX_LIBS += $(WATT32_ROOT)/lib/libwatt.a
 
-PROGRAM  = curl.exe
-OBJECTS += $(addprefix $(OBJ_DIR)/, $(CSOURCES:.c=.o))
+PROGRAM = curl.exe
+OBJECTS = $(addprefix $(OBJ_DIR)/, $(notdir $(CSOURCES:.c=.o)))
 
 all: $(OBJ_DIR) $(PROGRAM)
 	@echo Welcome to curl
@@ -64,10 +66,14 @@ $(PROGRAM): $(OBJECTS) ../lib/libcurl.a
 
 #
 # groff 1.18+ requires "-P -c"
+# If 'USE_ZLIB = 1', create a compressed help-file.
 #
-tool_hugehelp.c: ../docs/MANUAL ../docs/curl.1 mkhelp.pl
-	groff -Tascii -man ../docs/curl.1 | \
-	perl -w mkhelp.pl ../docs/MANUAL > $@
+ifeq ($(USE_ZLIB),1)
+  COMPRESS_OPT = -c
+endif
+
+tool_hugehelp.c: ../docs/curl.1 mkhelp.pl Makefile.dj
+	groff -Tascii -man $< | perl -w mkhelp.pl $(COMPRESS_OPT) $< > $@
 
 # clean generated files
 #