[CMake] ExternalProject breaks a parallel make.

[Andrey Nikitin]

Hi.

I have problem with make -jN and Cmake ExternalProject.
% alias make="make -j5"
% make
Scanning dependencies of target EXTLIB
[  0%] Creating directories for 'EXTLIB'
[  0%] No download step for 'EXTLIB'
[  0%] [  0%] No patch step for 'EXTLIB'
No update step for 'EXTLIB'
[  0%] No configure step for 'EXTLIB'
[  0%] Performing build step for 'EXTLIB'
make[3]: warning: jobserver unavailable: using -j1.  Add `+' to parent
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
         ^^^            THIS IS MY PROBLEM             ^^^
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 
make rule. start EXTLIB build: duration 5 sec. EXTLIB is built
[  0%] Performing install step for 'EXTLIB' EXTLIB *.{a,so} objects are
installed [  0%] No test step for 'EXTLIB'
[  0%] Completed 'EXTLIB'
[ 90%] Built target EXTLIB
Scanning dependencies of target hello
[100%] Building C object CMakeFiles/hello.dir/main.c.o
Linking C executable hello
[100%] Built target hello


My test project files contents see below.

% tree -AF --dirsfirst
.
├── build/
├── extlib/
│   ├── CMakeLists.txt
│   └── Makefile
├── CMakeLists.txt
└── main.c

% cat CMakeLists.txt 
cmake_minimum_required(VERSION 2.8)
project ( hello C )
add_subdirectory( extlib )
add_executable( ${PROJECT_NAME} WIN32 main.c )
add_dependencies(  ${PROJECT_NAME} EXTLIB )

% cat extlib/CMakeLists.txt 
cmake_minimum_required(VERSION 2.8)
include(ExternalProject)

ExternalProject_Add( EXTLIB
   SOURCE_DIR ${CMAKE_CURRENT_SOURCE_DIR}
   BUILD_IN_SOURCE 1
   DOWNLOAD_COMMAND ""
   UPDATE_COMMAND   ""
   PATCH_COMMAND   ""
   CONFIGURE_COMMAND  "" 
   # BUILD_COMMAND  ""
   TEST_COMMAND ""
   INSTALL_COMMAND echo "EXTLIB *.{a,so} objects are installed"
)

% cat extlib/Makefile 
all:
	echo "start EXTLIB build: duration 5 sec."
	sleep 5
	echo "EXTLIB is built"

% cat main.c 
#include <stdio.h>

int main( int argc, const char* argv[] )
{
   printf("Hello world!\n");
}

% cmake --version
cmake version 2.8.2 (Debian/GNU Linux 6.0.1)