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	2017-05-11 09:33:33 UTC
+++ libproxy/CMakeLists.txt
@@ -6,18 +6,19 @@ set_project_version(0 4 15)
 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)
 
 ### Modules
 include(cmake/modules.cmk)
 
-### Dev files
-include(cmake/devfiles.cmk)
+if(WITH_LIBRARY)
+    ### Dev files
+    include(cmake/devfiles.cmk)
 
-### Main library
-include(cmake/libproxy.cmk)
+    ### Main library
+    include(cmake/libproxy.cmk)
+endif()
 
 ### Tests
 add_testdirectory(test)