Makefile.mk: expect NROFF variable instead of auto-detection

Auto-detection used either `command -v` (for unixy shells) or `where`
(for Windows ones). The former is an internal command, and it fails to
run from an MSYS2 shell via a non-MSYS2 GNU Make, because make executes
it as-is via `CreateProcess()`. This also results in a warning.

Prepending `$(SHELL) -c` would fix it, but would also need to enclose
the rest of the command line in quotes, what makes this code
overly complex for little benefit. Using `which` would also solve it,
but this isn't the correct way to check for tool presence and some *nix
systems don't have it.

Resolve this by dropping auto-detection and expect the caller to set the
`NROFF` variable to `nroff`/`groff` tool, in case someone wants to
rebuild the manual and such tool is available.

Closes #10000

[ci skip]

lib/Makefile.mk

@@ -334,13 +334,11 @@ DEL   = rm -f $1
 COPY  = -cp -afv $1 $2
 MKDIR = mkdir -p $1
 RMDIR = rm -fr $1
-WHICH = command -v
 else
 DEL   = -del 2>NUL /q /f $(subst /,\,$1)
 COPY  = -copy 2>NUL /y $(subst /,\,$1) $(subst /,\,$2)
 MKDIR = -md 2>NUL $(subst /,\,$1)
 RMDIR = -rd 2>NUL /q /s $(subst /,\,$1)
-WHICH = where
 endif
 
 all: $(TARGETS)

src/Makefile.mk

@@ -80,11 +80,10 @@ TOCLEAN := $(curl_OBJECTS)
 
 ifneq ($(wildcard tool_hugehelp.c.cvs),)
 PERL  ?= perl
-NROFF ?= groff
 
 TOCLEAN += tool_hugehelp.c
 
-ifneq ($(shell $(WHICH) $(NROFF)),)
+ifdef NROFF
 $(PROOT)/docs/curl.1: $(wildcard $(PROOT)/docs/cmdline-opts/*.d)
 	cd $(PROOT)/docs/cmdline-opts && \
 	$(PERL) gen.pl mainpage $(notdir $^) > ../curl.1