[CMake] CheckLibraryExists requires C Compiler

[Hannes Hauswedell]

Dear CMake-Hackers,

I have a project which is purely C++. We have recently switched from

project (FOO)

to

project (FOO CXX)

so that we don't depend on CMAKE_C_COMPILER being set. This has broken
our BZip2 detection:

-- Found BZip2: /usr/lib/x86_64-linux-gnu/libbz2.so (found version "1.0.6")
-- Looking for BZ2_bzCompressInit in /usr/lib/x86_64-linux-gnu/libbz2.so
CMake Error at /usr/share/cmake-3.0/Modules/CheckLibraryExists.cmake:52
(try_compile):
  Unknown extension ".c" for file

    /usr/share/cmake-3.0/Modules/CheckFunctionExists.c

  try_compile() works only for enabled languages.  Currently these are:

    CXX

  See project() command to enable other languages.
Call Stack (most recent call first):
  /usr/share/cmake-3.0/Modules/FindBZip2.cmake:60 (CHECK_LIBRARY_EXISTS)

Note that this is fixed in cmake-3.4 by
https://github.com/Kitware/CMake/commit/23876eda9f1d4b5725407aef7d218fc3ce4113cb
because the CheckLibraryExists is removed. However I depend on shipping
the software on platforms with cmake>=3.0 so this doesn't help.

I have tried to set
SET_SOURCE_FILES_PROPERTIES(CheckFunctionExists.c PROPERTIES LANGUAGE CXX)
but this doesn't help, either.

Is there any workaround for this problem?

And isn't requiring C_COMPILER in CheckLibraryExists a bug? IMHO it
should check CMAKE_C_COMPILER_LOADED and use cxx otherwise...

Thank you for your help!
-- 
Best regards,
Hannes Hauswedell
Free Software Foundation Europe -- German Team

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