CMake: Re-work configure flags for CMake generators

Remove the -cmake-makefiles configure argument as its meaning was in
essence "do not pass a -G argument to CMake".
Instead, we add the following arguments:
    -cmake-generator <name> to pass -G <name> to CMake
    -cmake-use-default-generator to pass no -G argument to CMake

If none of those arguments is given, we try to autodetect the
generator. If a ninja executable is found, we prefer the Ninja
generator. On Unix we fall back to "Unix Makefiles".
On Windows, we do a poor man's compiler detection and select one of
"NMake Makefiles", "NMake Makefiles JOM" and "MinGW Makefiles".

Change-Id: Ic36669bd50956d15fbc71cee73720732cd4bfab8
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>

cmake/QtProcessConfigureArgs.cmake

@@ -29,13 +29,16 @@ get_filename_component(source_dir ".." ABSOLUTE BASE_DIR "${CMAKE_CURRENT_LIST_D
 file(STRINGS "${OPTFILE}" configure_args)
 list(FILTER configure_args EXCLUDE REGEX "^[ \t]*$")
 list(TRANSFORM configure_args STRIP)
-set(set_generator TRUE)
+unset(generator)
+set(auto_detect_generator TRUE)
 while(configure_args)
     list(POP_FRONT configure_args arg)
     if(arg STREQUAL "-cmake")
         # ignore
-    elseif(arg STREQUAL "-cmake-makefiles")
-        set(set_generator FALSE)
+    elseif(arg STREQUAL "-cmake-generator")
+        list(POP_FRONT configure_args generator)
+    elseif(arg STREQUAL "-cmake-use-default-generator")
+        set(auto_detect_generator FALSE)
     elseif(arg STREQUAL "-top-level")
         get_filename_component(source_dir "../.." ABSOLUTE BASE_DIR "${CMAKE_CURRENT_LIST_DIR}")
     elseif(arg STREQUAL "-skip")
@@ -101,8 +104,29 @@ while(configure_args)
     endif()
 endwhile()
 
-if(set_generator)
-    push(-G Ninja)
+if(NOT generator AND auto_detect_generator)
+    find_program(ninja ninja)
+    if(ninja)
+        set(generator Ninja)
+    else()
+        if(CMAKE_HOST_UNIX)
+            set(generator "Unix Makefiles")
+        elseif(CMAKE_HOST_WINDOWS)
+            find_program(msvc_compiler cl.exe)
+            if(msvc_compiler)
+                set(generator "NMake Makefiles")
+                find_program(jom jom)
+                if(jom)
+                    string(APPEND generator " JOM")
+                endif()
+            else()
+                set(generator "MinGW Makefiles")
+            endif()
+        endif()
+    endif()
+endif()
+if(generator)
+    push(-G "${generator}")
 endif()
 
 push("${source_dir}")

config_help.txt

@@ -85,6 +85,11 @@ Build options:
   -commercial .......... Build the Commercial Edition of Qt
   -confirm-license ..... Automatically acknowledge the license
 
+  -cmake ............... Use the CMake build system instead of the qmake one.
+  -cmake-generator <name> ... Explicitly specify the build system generator for
+                         CMake instead of auto-detecting one.
+  -cmake-use-default-generator ... Turn off auto-detection of the CMake build
+                         system generator.
   -release ............. Build Qt with debugging turned off [yes]
   -debug ............... Build Qt with debugging turned on [no]
   -debug-and-release ... Build two versions of Qt, with and without