[CMake] CMake performs search for includes/libs in non-default compiler search paths.

[arrowdodger]

Hello. On FreeBSD everything, that distributes with system goes to /usr (i
mean includes go to /usr/includes and libs to /usr/lib) and all 3d party
stuff goes to /usr/local.
And the compiler is intentionally set to look only in /usr/include. The same
is for linker - it's looking for libs only in /usr/lib by default. So, if
user want to use some 3d-party library, he should add -I/usr/local/include
and -L/usr/local/lib to build command.

Now, i'm using find_library() and find_path() to locate 3d-party library in
such way:

> find_path(FFI_INCLUDE_PATH ffi.h PATHS ${USER_DEFINED_INCLUDE_DIR})
> find_library(FFI_LIBRARY_PATH ffi PATHS ${USER_DEFINED_LIB_DIR})
>
I'm expecting that search will not succeed until i supply CMake with
additional directories to search. But it's succeeds because
CMAKE_SYSTEM_PREFIX_PATH from Modules/Platform/UnixPaths.cmake is set to
"/usr;/usr/local;/". And this file is included by
Modules/Platform/FreeBSD.cmake.

Later i'm doing:

> if(${USER_DEFINED_INCLUDE_DIR})
>   include_directories(${FFI_INCLUDE_DIR})
> endif()
>

On FreeBSD it leads to "No such file or directory: ffi.h" error. So here is
the question: Is this a CMake bug? Or it's wrong to assume that headers and
library are in compiler's default search path if CMake finds them without
user's help?
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