Determining if the PHYSFS_getPrefDir exist failed with the following output:
Change Dir: /home/peko/autobuild/instance-0/output-1/build/supertux-0.6.0/CMakeFiles/CMakeTmp

Run Build Command:"/usr/bin/make" "cmTC_d6a38/fast"
make[1]: Entering directory '/home/peko/autobuild/instance-0/output-1/build/supertux-0.6.0/CMakeFiles/CMakeTmp'
/usr/bin/make -f CMakeFiles/cmTC_d6a38.dir/build.make CMakeFiles/cmTC_d6a38.dir/build
make[2]: Entering directory '/home/peko/autobuild/instance-0/output-1/build/supertux-0.6.0/CMakeFiles/CMakeTmp'
Building C object CMakeFiles/cmTC_d6a38.dir/CheckSymbolExists.c.o
/home/peko/autobuild/instance-0/output-1/host/bin/x86_64-linux-gcc --sysroot=/home/peko/autobuild/instance-0/output-1/host/x86_64-buildroot-linux-musl/sysroot   -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os   -DNDEBUG   -o CMakeFiles/cmTC_d6a38.dir/CheckSymbolExists.c.o   -c /home/peko/autobuild/instance-0/output-1/build/supertux-0.6.0/CMakeFiles/CMakeTmp/CheckSymbolExists.c
Linking C executable cmTC_d6a38
/usr/bin/cmake -E cmake_link_script CMakeFiles/cmTC_d6a38.dir/link.txt --verbose=1
/home/peko/autobuild/instance-0/output-1/host/bin/x86_64-linux-gcc --sysroot=/home/peko/autobuild/instance-0/output-1/host/x86_64-buildroot-linux-musl/sysroot -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os   -DNDEBUG    CMakeFiles/cmTC_d6a38.dir/CheckSymbolExists.c.o  -o cmTC_d6a38 
/home/peko/autobuild/instance-0/output-1/host/opt/ext-toolchain/bin/../lib/gcc/x86_64-buildroot-linux-musl/8.3.0/../../../../x86_64-buildroot-linux-musl/bin/ld: CMakeFiles/cmTC_d6a38.dir/CheckSymbolExists.c.o: in function `main':
CheckSymbolExists.c:(.text.startup+0x6): undefined reference to `PHYSFS_getPrefDir'
collect2: error: ld returned 1 exit status
make[2]: *** [CMakeFiles/cmTC_d6a38.dir/build.make:87: cmTC_d6a38] Error 1
make[2]: Leaving directory '/home/peko/autobuild/instance-0/output-1/build/supertux-0.6.0/CMakeFiles/CMakeTmp'
make[1]: *** [Makefile:121: cmTC_d6a38/fast] Error 2
make[1]: Leaving directory '/home/peko/autobuild/instance-0/output-1/build/supertux-0.6.0/CMakeFiles/CMakeTmp'

File /home/peko/autobuild/instance-0/output-1/build/supertux-0.6.0/CMakeFiles/CMakeTmp/CheckSymbolExists.c:
/* */
#include </home/peko/autobuild/instance-0/output-1/host/x86_64-buildroot-linux-musl/sysroot/usr/include/physfs.h>

int main(int argc, char** argv)
{
  (void)argv;
#ifndef PHYSFS_getPrefDir
  return ((int*)(&PHYSFS_getPrefDir))[argc];
#else
  (void)argc;
  return 0;
#endif
}

Performing C++ SOURCE FILE Test HAVE_ICONV_CONST failed with the following output:
Change Dir: /home/peko/autobuild/instance-0/output-1/build/supertux-0.6.0/CMakeFiles/CMakeTmp

Run Build Command:"/usr/bin/make" "cmTC_424db/fast"
make[1]: Entering directory '/home/peko/autobuild/instance-0/output-1/build/supertux-0.6.0/CMakeFiles/CMakeTmp'
/usr/bin/make -f CMakeFiles/cmTC_424db.dir/build.make CMakeFiles/cmTC_424db.dir/build
make[2]: Entering directory '/home/peko/autobuild/instance-0/output-1/build/supertux-0.6.0/CMakeFiles/CMakeTmp'
Building CXX object CMakeFiles/cmTC_424db.dir/src.cxx.o
/home/peko/autobuild/instance-0/output-1/host/bin/x86_64-linux-g++ --sysroot=/home/peko/autobuild/instance-0/output-1/host/x86_64-buildroot-linux-musl/sysroot    -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64  -Os   -DGLEW_NO_GLU -DHAVE_ICONV_CONST  -DNDEBUG   -o CMakeFiles/cmTC_424db.dir/src.cxx.o -c /home/peko/autobuild/instance-0/output-1/build/supertux-0.6.0/CMakeFiles/CMakeTmp/src.cxx
/home/peko/autobuild/instance-0/output-1/build/supertux-0.6.0/CMakeFiles/CMakeTmp/src.cxx:4:9: error: ambiguating new declaration of 'double iconv(iconv_t, char**, size_t*, char**, size_t*)'
  double iconv(iconv_t cd,  char **inbuf, size_t *inbytesleft, char **outbuf, size_t *outbytesleft);
         ^~~~~
In file included from /home/peko/autobuild/instance-0/output-1/build/supertux-0.6.0/CMakeFiles/CMakeTmp/src.cxx:2:
/home/peko/autobuild/instance-0/output-1/host/x86_64-buildroot-linux-musl/sysroot/usr/include/iconv.h:17:8: note: old declaration 'size_t iconv(iconv_t, char**, size_t*, char**, size_t*)'
 size_t iconv(iconv_t, char **__restrict, size_t *__restrict, char **__restrict, size_t *__restrict);
        ^~~~~
make[2]: *** [CMakeFiles/cmTC_424db.dir/build.make:66: CMakeFiles/cmTC_424db.dir/src.cxx.o] Error 1
make[2]: Leaving directory '/home/peko/autobuild/instance-0/output-1/build/supertux-0.6.0/CMakeFiles/CMakeTmp'
make[1]: *** [Makefile:121: cmTC_424db/fast] Error 2
make[1]: Leaving directory '/home/peko/autobuild/instance-0/output-1/build/supertux-0.6.0/CMakeFiles/CMakeTmp'

Source file was:

	#include <iconv.h>
	// this declaration will fail when there already exists a non const char** version which returns size_t
	double iconv(iconv_t cd,  char **inbuf, size_t *inbytesleft, char **outbuf, size_t *outbytesleft);
	int main() { return 0; }