Print a very useful diagnostic when cpp isn't available

This is crucial to determining if the pure-Perl pre-processing is
screwing up or if it's a bug parsing the CPP output.

The line will be especially useful to see in bug reports.
Strip the intervening space in enum declarations when under an ISO-compliant CPP

This commit resolves issues with constants being missed and tests
failing horribly because of missing constants like CURLFORM_ARRAY
(usually the first failure).

Since 2001, curl.h has used a macro (CFINIT) which generates enums like:

    CURLFORM_ ARRAY,

when running under an ISO-compliant cpp.  Under a non-compliant cpp, the
space is omitted using /**/ (see curl.h), and the pure-Perl parsing of
curl.h already handles the CFINIT constructs correctly.

On my test system, the following cpp uses the ISO-compliant form and
thus this module fails to build properly and test:

    Apple LLVM version 4.2 (clang-425.0.28) (based on LLVM 3.2svn)
    Target: x86_64-apple-darwin12.5.0
    Thread model: posix
Merge pull request #7 from tsibley/isocpp-defines

Properly parse the CFINIT macro output under CURL_ISOCPP