document the gcc requirement

CONTRIBUTING.rst

@@ -12,6 +12,9 @@ the website. You can also write to <varnish-support@uplex.de>.
 For developers
 --------------
 
+This project requires use of the gcc compiler, since the RHash sources
+require it.
+
 The build specifies C99 conformance, all compiler warnings turned on,
 and all warnings considered errors (compiler options ``-std=c99
 -Werror -Wall``).

README.rst

@@ -481,9 +481,9 @@ REQUIREMENTS
 This VMOD requires at least Varnish version 4.1, and has been tested
 against versions through 5.0.0.
 
-Perl 5 is required for the build. For the self-tests invoked by ``make
-check``, the VMODs ``blobcode`` and ``blob`` must be installed (see
-`SEE ALSO`_).
+The gcc compiler and Perl 5 are required for the build. For the
+self-tests invoked by ``make check``, the VMODs ``blobcode`` and
+``blob`` must be installed (see `SEE ALSO`_).
 
 LIMITATIONS
 ===========

src/vmod_blobdigest.vcc

@@ -421,9 +421,9 @@ REQUIREMENTS
 This VMOD requires at least Varnish version 4.1, and has been tested
 against versions through 5.0.0.
 
-Perl 5 is required for the build. For the self-tests invoked by ``make
-check``, the VMODs ``blobcode`` and ``blob`` must be installed (see
-`SEE ALSO`_).
+The gcc compiler and Perl 5 are required for the build. For the
+self-tests invoked by ``make check``, the VMODs ``blobcode`` and
+``blob`` must be installed (see `SEE ALSO`_).
 
 LIMITATIONS
 ===========