cmake: fix ucontext detection

On some archs uclibc does not provide the ucontext structure despite
providing ucontext.h, for details see
https://git.buildroot.net/buildroot/commit/?id=f1cbfeea95e6287c7a666aafc182ffa318eff262

This patch improves the detection of ucontext by making sure that
HAVE_UCONTEXT_H is only set when makecontext() was found.
Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>

configure.cmake

@@ -1107,11 +1107,11 @@ SET(CMAKE_EXTRA_INCLUDE_FILES)
 CHECK_STRUCT_HAS_MEMBER("struct dirent" d_ino "dirent.h"  STRUCT_DIRENT_HAS_D_INO)
 CHECK_STRUCT_HAS_MEMBER("struct dirent" d_namlen "dirent.h"  STRUCT_DIRENT_HAS_D_NAMLEN)
 SET(SPRINTF_RETURNS_INT 1)
-CHECK_INCLUDE_FILE(ucontext.h HAVE_UCONTEXT_H)
-IF(NOT HAVE_UCONTEXT_H)
-  CHECK_INCLUDE_FILE(sys/ucontext.h HAVE_UCONTEXT_H)
+CHECK_INCLUDE_FILE(ucontext.h HAVE_FILE_UCONTEXT_H)
+IF(NOT HAVE_FILE_UCONTEXT_H)
+  CHECK_INCLUDE_FILE(sys/ucontext.h HAVE_FILE_UCONTEXT_H)
 ENDIF()
-IF(HAVE_UCONTEXT_H)
+IF(HAVE_FILE_UCONTEXT_H)
   CHECK_FUNCTION_EXISTS(makecontext HAVE_UCONTEXT_H)
 ENDIF()