grovel: rename pkg-config to pkg-config-cflags

Also, by default failure to execute pkg-config is fatal and it takes a
single pkg rather than a list.

doc/cffi-manual.texinfo

@@ -6489,15 +6489,18 @@ Adds @var{cc-flags} to the command line arguments used for the C compiler
 invocation.
 @end deffn
 
-@deffn {Grovel Form} pkg-flags &rest pkgs
-
-Adds @var{pkgs} to the command line arguments for the external program
-pkg-config and runs it to retrieve the relevant include flags used for the
-C compiler invocation. This syntax can be used instead of hard-coding
-paths using @code{cc-flags}, and ensures that include flags are added
-correctly on the build system. Assumes pkg-config is installed and working.
-@var{pkgs} are strings that identify installed pkg-config packages. See
-the pkg-config manual for more information.
+@deffn {Grovel Form} pkg-config-cflags pkg &key optional
+
+Adds @var{pkg} to the command line arguments for the external program
+@code{pkg-config} and runs it to retrieve the relevant include flags
+used for the C compiler invocation. This syntax can be used instead of
+hard-coding paths using @code{cc-flags}, and ensures that include
+flags are added correctly on the build system. Assumes
+@code{pkg-config} is installed and working.  @var{pkg} is a string
+that identifies an installed @code{pkg-config} package. See the
+pkg-config manual for more information. If @var{optional} is true,
+failure to execute @code{pkg-config} does @emph{not} abort
+compilation.
 @end deffn
 
 @deffn {Grovel Form} cstruct lisp-name c-name slots

grovel/grovel.lisp

@@ -334,15 +334,16 @@ int main(int argc, char**argv) {
 (define-grovel-syntax cc-flags (&rest flags)
   (appendf *cc-flags* (trim-whitespace flags)))
 
-(define-grovel-syntax pkg-flags (&rest pkgs)
-  (handler-case
+(define-grovel-syntax pkg-config-cflags (pkg &key optional)
+  (block nil
+    (handler-bind
+        ((error (lambda (e)
+                  (when optional
+                    (format *debug-io* "~&ERROR: ~a" e)
+                    (format *debug-io* "~&Attempting to continue anyway.~%")
+                    (return)))))
       (appendf *cc-flags*
-               (trim-whitespace (mapcar #'(lambda (p)
-                                            (invoke "pkg-config" p "--cflags"))
-                                        (trim-whitespace pkgs))))
-    (error (e)
-      (format *debug-io* "~&ERROR: ~a" e)
-      (format *debug-io* "~&Attempting to continue anyway.~%"))))
+               (trim-whitespace (list (invoke "pkg-config" pkg "--cflags")))))))
 
 ;;; This form also has some "read time" effects. See GENERATE-C-FILE.
 (define-grovel-syntax in-package (name)

libffi/libffi-unix.lisp

@@ -38,8 +38,7 @@
 #+openbsd
 (cc-flags "-I/usr/local/include")
 
-#+linux
-(pkg-flags "libffi")
+(pkg-config-cflags "libffi" :optional t)
 
 #+darwin
 (include "ffi/ffi.h")