cmake: clean OtherTests, fixing -Werror

There were several -Wunused warnings and one duplicate macro definition.
The EXTRA_DEFINES variable of the CurlCheckCSources macro was being
abused ("__unused1\n#undef inline\n#define __unused2", seriously?) to
insert extra C code. Avoid this broken abstraction and use cmake's
check_c_source_compiles directly (works fine with CMake 2.8, maybe
even cmake 2.6).

After cleaning up all related variables (EXTRA_DEFINES,
HEADER_INCLUDES, auxiliary headers_hack), also remove a duplicate
add_headers_include macro and remove duplicate header additions before
the struct timeval check.

Oh, and now the code is converted to use CheckCSourceRuns and
CheckCSourceCompiles, the two curl-specific helpers can be removed.
Unfortunately, the cmake output is now slightly more verbose. Before:

    Performing Test int send(int, const void *, size_t, int) (curl_cv_func_send_test)
    Performing Test int send(int, const void *, size_t, int) (curl_cv_func_send_test) - Failed

Since check_c_source_compiles prints the varname, now you see:

    Performing Test curl_cv_func_send_test
    Performing Test curl_cv_func_send_test - Failed
    Tested: int send(int, const void *, size_t, int)

Compared cmake output with each other using vimdiff, no functional
differences were found. Tested with GCC 4.9.1 and Clang 3.5.0.

Signed-off-by: Peter Wu <peter@lekensteyn.nl>