[Bug preprocessor/64864] New: [5 Regression] preprocessor linemarkers break configure checks

["trippels at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64864

            Bug ID: 64864
           Summary: [5 Regression] preprocessor linemarkers break
                    configure checks
           Product: gcc
           Version: 5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: preprocessor
          Assignee: unassigned at gcc dot gnu.org
          Reporter: trippels at gcc dot gnu.org
                CC: dodji at gcc dot gnu.org

For example (this happens for many projects that check for the boost version):
...
checking for working strtod... yes
checking for gettimeofday... yes
checking for Boost headers version >= 1.36.0... yes
checking for Boost's header version... 
configure: error: invalid value: boost_major_version=

markus@x4 core % cat test.cpp
#include <boost/version.hpp>                                                    
boost-lib-version = BOOST_LIB_VERSION

markus@x4 core % g++ -E test.cpp
# 1 "test.cpp"
# 1 "<built-in>"
# 1 "<command-line>"
# 1 "/usr/include/stdc-predef.h" 1 3 4
# 1 "<command-line>" 2
# 1 "test.cpp"
# 1 "/usr/include/boost/version.hpp" 1 3 4
# 2 "test.cpp" 2
boost-lib-version = 
# 2 "test.cpp" 3 4
                   "1_56"

markus@x4 core % /usr/x86_64-pc-linux-gnu/gcc-bin/4.9.2/g++ -E test.cpp
# 1 "test.cpp"
# 1 "<built-in>"
# 1 "<command-line>"
# 1 "/usr/include/stdc-predef.h" 1 3 4
# 1 "<command-line>" 2
# 1 "test.cpp"
# 1 "/usr/include/boost/version.hpp" 1 3 4
# 2 "test.cpp" 2
boost-lib-version = "1_56"

I know that these linemarkers are valid. 
But is it really necessary that they appear in the middle of statements? 

Using -P is a workaround, that apparently nobody uses in configure scripts.

See also PR64604.