CMake: Test for & set WORDS_BIGENDIAN, set _GNU_SOURCE.

Got lost in transition from autotools.

CMakeLists.txt

@@ -48,6 +48,7 @@ include (GncAddGSchemaTargets)
 include (GncAddTest)
 include (MakeDistFiles)
 include (GNUInstallDirs)
+include (TestBigEndian)
 
 # ############################################################
 # These options are settable from the CMake command line. For example, to disable
@@ -545,6 +546,7 @@ check_c_compiler_flag(-Wstringop-truncation have_stringop_truncation)
 if (have_stringop_truncation)
     set(HAVE_STRINGOP_TRUNCATION TRUE)
 endif()
+add_definitions(-D_GNU_SOURCE)
 
 if (APPLE)
   include (CheckCXXCompilerFlag)
@@ -688,6 +690,11 @@ check_include_files (unistd.h HAVE_UNISTD_H)
 check_include_files (utmp.h HAVE_UTMP_H)
 check_include_files (wctype.h HAVE_WCTYPE_H)
 
+test_big_endian(IS_BIGENDIAN)
+if (IS_BIGENDIAN)
+  set(WORDS_BIGENDIAN)
+endif (IS_BIGENDIAN)
+
 if (NOT DISABLE_NLS)
 set(ENABLE_NLS 1)
 endif(NOT DISABLE_NLS)