[cmake-developers] Patch for FindBZip2.cmake BZIP2_NEED_PREFIX issue with cross compiled mingw32 applications

[Ralf Habacker]

Hi,

the appended patch fixes an issue I have with recent cmake (3.2.3) with
cross compiling an mingw32 application. The applications requires bzip2
library which is handled by cmake provided FindBZip2.cmake. The problem
is that  BZIP2_NEED_PREFIX  is not set where it should be and results
into linker failure not finding bzip2 related symbols.

Some details:

FindBZip2.cmake uses BZ2_bzCompressInit to detect if BZIP2_NEED_PREFIX
should be set.

   CHECK_LIBRARY_EXISTS("${BZIP2_LIBRARIES}" BZ2_bzCompressInit ""
BZIP2_NEED_PREFIX)

The check is performed with CheckFunctionExists.c, which converts the
function name into a prototype of the form 'char BZ2_bzCompressInit()',
while in real it is

int BZ2_bzCompressInit ( bz_stream *strm, 
                         int blockSize100k, 
                         int verbosity,
                         int workFactor );


The compiler converts this function to the symbol '_BZ2_bzCompressInit',
which is not in the related import library. 
Taking a look into the related import library shows that there is

objdump -x /usr/i686-w64-mingw32/sys-root/mingw/lib/libbz2.dll.a  | grep
CompressInit
[  6](sec  1)(fl 0x00)(ty   0)(scl   2) (nx 0) 0x00000000
_BZ2_bzCompressInit at 16
[  7](sec  3)(fl 0x00)(ty   0)(scl   2) (nx 0) 0x00000000
__imp__BZ2_bzCompressInit at 16

The  '@16' indicates the size of the parameter list
(https://msdn.microsoft.com/en-us/library/zxk0tw93.aspx) with does match
the used prototype 'char BZ2_bzCompressInit().'

The fix is to use a function which do not have parameters like
BZ2_decompress.

objdump -x /usr/i686-w64-mingw32/sys-root/mingw/lib/libbz2.dll.a  | grep
decompress
[  6](sec  1)(fl 0x00)(ty   0)(scl   2) (nx 0) 0x00000000 _BZ2_decompress
[  7](sec  3)(fl 0x00)(ty   0)(scl   2) (nx 0) 0x00000000
__imp__BZ2_decompress

The appended patch fixes this issue.

Regards
 Ralf

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