gcc: Fix/rework the intro.

* doc/chapters/c-extensions.texi: Move `--disable-gcc-extensions' to a
  footnote.  Rephrase the first sentence.

doc/chapters/c-extensions.texi

@@ -7,13 +7,11 @@
 @cindex C extensions
 @cindex GCC plug-in
 
-If the GCC used to compile StarPU provides a plug-in
-support@footnote{This feature is only available for GCC 4.5 and
-later.}, StarPU will then build a plug-in for the GNU Compiler
-Collection (GCC), which defines extensions to languages of the C
-family (C, C++, Objective-C) that make it easier to write StarPU code.
-
-The plug-in can be disabled by configuring with @code{--disable-gcc-extensions}.
+When GCC plug-in support is available, StarPU builds a plug-in for the
+GNU Compiler Collection (GCC), which defines extensions to languages of
+the C family (C, C++, Objective-C) that make it easier to write StarPU
+code@footnote{This feature is only available for GCC 4.5 and later.  It
+can be disabled by configuring with @code{--disable-gcc-extensions}.}.
 
 Those extensions include syntactic sugar for defining
 tasks and their implementations, invoking a task, and manipulating data