CMakeParseImplicitLinkInfo: Fix implicit library logging

In commit v3.1.0-rc1~640^2~5 (Clean up usage of if(... MATCHES regex)
followed string(REGEX REPLACE regex), 2014-04-06) we accidentally broke
logging of the implicit library detection.  Restore use of the
intermediate 'lib' variable so that the log message is constructed
properly.

Reported-by: Bill Somerville <bill@classdesign.com>
Makefile: Do not create an empty linker response file

Since commit v3.1.0-rc1~821^2 (Windows: Use response files to specify
link libraries for GNU tools, 2014-03-04) we use a response file to pass
possibly long linker flag lists to the GNU linker on Windows.  On MinGW,
this may cause gfortran to use a response file to pass some flags to its
own internal invocation.  This is okay except when we are parsing
implicit link flags from the compiler ABI detection build.  If gfortran
uses a response file in that case then we may miss extracting some of
the implicit link flags, such as -lgfortran.  Fortunately, in the
compiler ABI detection case we do not actually link to anything so the
response file is empty.  Work around this problem by simply not using a
response file when the list of flags it is used to pass is empty (or
just whitespace).

Reported-by: Bill Somerville <bill@classdesign.com>
Merge branch 'fix-implicit-lib-logging' into release
Merge branch 'link-no-empty-response-files' into release
Help: Fix typo in cmake-compile-features(7) manual

Fix spelling 'execptions' => 'exceptions'.
Merge branch 'doc-compile-features-typo' into release