Only enable the test when using GNU make

The makefile used in the test uses $(shell ...), which is
AFAIK a GNU extension, and will probably not work e.g. with OpenBSD make.
According to the FreeBSD make manpage their make has a != assignment,
which seems to do something similar, but I don't have such a system
around for testing.
Also, the point of this test is not to write a portable makefile,
but to check whether cmake --find-package prints a correct string.

Alex

Tests/FindPackageModeMakefileTest/CMakeLists.txt

@@ -1,20 +1,30 @@
 
 
-# the test program links against the png lib, so test first whether it exists
 if(UNIX  AND  "${CMAKE_GENERATOR}" MATCHES "Makefile")
 
-  # build a library which we can search during the test
-  add_library(foo STATIC foo.cpp)
+  # Test whether the make is GNU make, and only add the test in this case,
+  # since the configured makefile in this test uses $(shell ...), which
+  # is AFAIK a GNU make extension. Alex
+  execute_process(COMMAND ${CMAKE_MAKE_PROGRAM} -v
+                  OUTPUT_VARIABLE makeVersionOutput
+                  TIMEOUT 10)
+  string(TOUPPER "${makeVersionOutput}" MAKE_VERSION_OUTPUT)
+  if("${MAKE_VERSION_OUTPUT}" MATCHES ".*GNU MAKE.*")
 
-  # configure a FindFoo.cmake so it knows where the library can be found
-  configure_file(FindFoo.cmake.in ${CMAKE_CURRENT_BINARY_DIR}/FindFoo.cmake @ONLY)
+    # build a library which we can search during the test
+    add_library(foo STATIC foo.cpp)
 
-  # now set up the test:
-  get_target_property(cmakeExecutable cmake LOCATION)
+    # configure a FindFoo.cmake so it knows where the library can be found
+    configure_file(FindFoo.cmake.in ${CMAKE_CURRENT_BINARY_DIR}/FindFoo.cmake @ONLY)
 
-  configure_file(${CMAKE_CURRENT_SOURCE_DIR}/Makefile.in ${CMAKE_CURRENT_BINARY_DIR}/ConfMakefile @ONLY)
-  configure_file(${CMAKE_CURRENT_SOURCE_DIR}/main.cpp ${CMAKE_CURRENT_BINARY_DIR}/main.cpp COPYONLY)
+    # now set up the test:
+    get_target_property(cmakeExecutable cmake LOCATION)
 
-  add_test(FindPackageModeMakefileTest ${CMAKE_MAKE_PROGRAM} -f ${CMAKE_CURRENT_BINARY_DIR}/ConfMakefile )
+    configure_file(${CMAKE_CURRENT_SOURCE_DIR}/Makefile.in ${CMAKE_CURRENT_BINARY_DIR}/ConfMakefile @ONLY)
+    configure_file(${CMAKE_CURRENT_SOURCE_DIR}/main.cpp ${CMAKE_CURRENT_BINARY_DIR}/main.cpp COPYONLY)
+
+    add_test(FindPackageModeMakefileTest ${CMAKE_MAKE_PROGRAM} -f ${CMAKE_CURRENT_BINARY_DIR}/ConfMakefile )
+
+  endif()
 
 endif()