Revert "compat/strdup.h: move common compat check for strdup() to own file"

This reverts commit aaba8c1080.

This commit breaks builddir != srcdir build, but, more importantly, it
also adds a dependency on a header, "config.h", which is not installed
and it's supposed to be private — since it's generated at configuration
time and it's not meant to be used by projects compiling against a
library.

CMakeLists.txt

@@ -23,7 +23,6 @@ endif()
 include_directories(${CMAKE_CURRENT_BINARY_DIR}/include)
 
 set(JSON_C_HEADERS
-    ./compat/strdup.h
     ./json.h
     ${CMAKE_CURRENT_BINARY_DIR}/include/config.h
     ./json_config.h