[CMake] linker flags for FreeBSD

[Brad King]

Alexander Neundorf wrote:
> when creating a shared library on FreeBSD with the flags 
> "-Wl,--fatal-warnings -Wl,--no-undefined" 
> I get undefined reference to everything from libc. Adding "-lc" helps. So 
> shouldn't "-lc" be added automatically on FreeBSD for linking shared libs 
> and modules ? 

I've now investigated this issue.  It appears to happen on other UNIX 
platforms such as Sun too.  This is basically the same problem as adding 
-lm on UNIX platforms if one uses any math symbols.  Since it is 
perfectly valid to create a shared library or module that does not 
reference any standard C library symbols, we do not want to force -lc on 
the users.  If a particular platform does force this option (such as 
Linux) then that is a problem for the users of that platform if they 
don't want it.  Therefore the resolution of this problem is just to 
require the CMakeLists.txt code to add -lc if it is building a library 
that references C library symbols:

IF(UNIX)
   SET(MY_LIBC_FLAGS "-lc")
ELSE(UNIX)
   SET(MY_LIBC_FLAGS "")
ENDIF(UNIX)

-Brad