Setting _POSIX_C_SOURCE unsets __BSD_VISIBLE by default, and several symbols
end up being hidden, leading to failures such as:

In file included from /wrkdir/libproxy-0.4.12/libproxy/test/url-encode.cpp:2:
In file included from /usr/include/c++/v1/iostream:38:
In file included from /usr/include/c++/v1/ios:216:
/usr/include/c++/v1/__locale:545:16: error: use of undeclared identifier 'isascii'
        return isascii(__c) ? (__tab_[static_cast<int>(__c)] & __m) !=0 : false;
               ^
/usr/include/c++/v1/__locale:552:22: error: use of undeclared identifier 'isascii'
            *__vec = isascii(*__low) ? __tab_[static_cast<int>(*__low)] : 0;
                     ^
/usr/include/c++/v1/__locale:560:17: error: use of undeclared identifier 'isascii'
            if (isascii(*__low) && (__tab_[static_cast<int>(*__low)] & __m))
                ^
/usr/include/c++/v1/__locale:569:19: error: use of undeclared identifier 'isascii'
            if (!(isascii(*__low) && (__tab_[static_cast<int>(*__low)] & __m)))
                  ^
--- libproxy/CMakeLists.txt.orig	2016-01-12 16:41:07 UTC
+++ libproxy/CMakeLists.txt
@@ -6,7 +6,6 @@ set_project_version(0 4 12)
 if (WIN32)
   add_definitions(-D_CRT_SECURE_NO_WARNINGS=1)
 else(WIN32)
-  add_definitions(-D_POSIX_C_SOURCE=200112L)
   set(CMAKE_CXX_FLAGS "-fvisibility=hidden ${CMAKE_CXX_FLAGS}")
 endif(WIN32)