package/ffmpeg: fix fenv build failure on ARCv2 with glibc

https://git.buildroot.net/buildroot/commit/toolchain?id=0633eb58a2912328508a76f8fe9875a0d113a880
added glibc support for ARCv2 which lacks fenv.h.

Code format was suggested by Arnout:
http://lists.busybox.net/pipermail/buildroot/2017-November/206631.html

Fixes
http://autobuild.buildroot.net/results/aec/aec300d7d4472421398a24202d6871c965bd1be5/

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
[Thomas: adjust how the Config.in dependency is expressed.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/ffmpeg/Config.in

@@ -1,6 +1,8 @@
 config BR2_PACKAGE_FFMPEG_ARCH_SUPPORTS
 	bool
 	default y
+	# fenv.h lacks FE_INVALID, FE_OVERFLOW & FE_UNDERFLOW
+	depends on !(BR2_archs38 && BR2_TOOLCHAIN_USES_GLIBC)
 	# fenv.h lacks FE_INVALID, FE_OVERFLOW & FE_UNDERFLOW on nios2
 	depends on !BR2_nios
 	# No support for ARMv7-M in the ARM assembly logic