cmake: when USE_MANUAL=YES, build the curl.1 man page

Fixes KNOWN_BUG 15.4

Closes #12742

docs/KNOWN_BUGS

@@ -92,7 +92,6 @@ problems may have been fixed or changed somewhat since this was written.
  15.1 cmake outputs: no version information available
  15.2 support build with GnuTLS
  15.3 unusable tool_hugehelp.c with MinGW
- 15.4 build docs/curl.1
  15.6 uses -lpthread instead of Threads::Threads
  15.7 generated .pc file contains strange entries
  15.8 libcurl.pc uses absolute library paths
@@ -528,12 +527,6 @@ problems may have been fixed or changed somewhat since this was written.
 
  see https://github.com/curl/curl/issues/3125
 
-15.4 build docs/curl.1
-
- The cmake build does not create the docs/curl.1 file and therefore must rely on
- it being there already. This makes the --manual option not work and test
- cases like 1139 cannot function.
-
 15.6 uses -lpthread instead of Threads::Threads
 
  See https://github.com/curl/curl/issues/6166

docs/cmdline-opts/CMakeLists.txt

@@ -28,8 +28,7 @@ transform_makefile_inc("Makefile.inc" "${CMAKE_CURRENT_BINARY_DIR}/Makefile.inc.
 include("${CMAKE_CURRENT_BINARY_DIR}/Makefile.inc.cmake")
 
 add_custom_command(OUTPUT "${MANPAGE}"
-  COMMAND "${PERL_EXECUTABLE}" "${CMAKE_CURRENT_SOURCE_DIR}/gen.pl" mainpage "${CMAKE_CURRENT_SOURCE_DIR}" > "${MANPAGE}"
-  DEPENDS ${DPAGES} ${OTHERPAGES}
+  COMMAND cd ${CMAKE_CURRENT_SOURCE_DIR} && "${PERL_EXECUTABLE}" "./gen.pl" mainpage ${DPAGES} > "${MANPAGE}"
   VERBATIM
 )
 add_custom_target(generate-curl.1 DEPENDS "${MANPAGE}")