Include <fpu_control.h> only on glibc systems

The <fpu_control.h> header provides _FPU_*() macros, which the
Init_FPU() function uses only on i386, and only when they are
defined. Since <fpu_control.h> is not available, this causes a build
failure with C libraries such as musl. By simply including
<fpu_control.h> explicitly on glibc systems, we avoid the build
failure, since mpcenc.c already conditionally uses the <fpu_control.h>
macros only if they are defined.

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
[Thomas: better commit log.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

diff -uNr musepack_src_r475.org/mpcenc/mpcenc.h musepack_src_r475/mpcenc/mpcenc.h
--- musepack_src_r475.org/mpcenc/mpcenc.h	2009-02-23 19:15:46.000000000 +0100
+++ musepack_src_r475/mpcenc/mpcenc.h	2016-01-30 09:29:08.000000000 +0100
@@ -50,7 +51,7 @@
 # include <unistd.h>
 #endif
 
-#if   defined __linux__
+#if   defined __GLIBC__
 #  include <fpu_control.h>
 #elif defined __FreeBSD__
 # include <machine/floatingpoint.h>