Fix compiling if no endian.h found

f7d65091 introduces error when no endian.h found in the system.

Since "CHECK_INCLUDE_FILE" returns empty string instead of "0", when
"cmake_config.h" is generated it has "#define CMAKE_HAVE_ENDIAN_H " line.
Later we have "#define HAVE_ENDIAN_H CMAKE_HAVE_ENDIAN_H" in the
"config.h", an thus "HAVE_ENDIAN_H" is also empty sting. Because of
this, "#if HAVE_ENDIAN_H" is incorrect preprocessor directive.

Signed-off-by: Craig Robbins <kde.psych@gmail.com>