Use C++ to test for int/size_t test. C can't distinct between those two.
https://bugs.gentoo.org/652054
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 37e9e87..5de2711 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -274,11 +274,11 @@ SET(CMAKE_REQUIRED_FLAGS "-Werror ${Guile_CFLAGS}")
 SET(CMAKE_REQUIRED_INCLUDES ${Guile_INCLUDE_DIRS})
 SET(CMAKE_REQUIRED_LIBRARIES ${Guile_LIBRARIES})
 
 MESSAGE(STATUS "Checking the size_t of guile strings")
 
-CHECK_C_SOURCE_COMPILES(  "#include <guile/gh.h>
+CHECK_CXX_SOURCE_COMPILES(  "#include <guile/gh.h>
   #include <libguile.h>
   void print_string (SCM s) {
     int len_r;
     char* r= gh_scm2newstr (s, &len_r); } int main() { return 0; }
 " _guile_str_size_test)