Prevent implicit conversion of float to double to avoid precision
error on i686.

  https://github.com/Maratyszcza/FP16/issues/20

Taken from Debian:

  https://salsa.debian.org/deeplearning-team/fp16/-/blob/master/debian/patches/ftbfs-i386.patch

Index: fp16/include/fp16/fp16.h
===================================================================
--- fp16.orig/include/fp16/fp16.h
+++ fp16/include/fp16/fp16.h
@@ -228,7 +228,8 @@ static inline uint16_t fp16_ieee_from_fp
 	const float scale_to_inf = fp32_from_bits(UINT32_C(0x77800000));
 	const float scale_to_zero = fp32_from_bits(UINT32_C(0x08800000));
 #endif
-	float base = (fabsf(f) * scale_to_inf) * scale_to_zero;
+	const volatile float base_inf = fabsf(f) * scale_to_inf;
+	float base = base_inf * scale_to_zero;
 
 	const uint32_t w = fp32_to_bits(f);
 	const uint32_t shl1_w = w + w;
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<tr><td><span title='2020-05-04 09:56:12 +0200'>2020-05-04</span></td><td><a href='/guix/commit/tests/dsa.key?id=43408e304ffb1149f35cb539b40d673d567c9116'>Add (guix openpgp).</a><span class='msg-avail'>...<span class='msg-tooltip'>* guix/openpgp.scm, tests/openpgp.scm, tests/civodul.key,
tests/dsa.key, tests/ed25519.key, tests/rsa.key,
tests/ed25519.sec: New files.
* Makefile.am (MODULES): Add guix/openpgp.scm.
(SCM_TESTS): Add tests/openpgp.scm.
(EXTRA_DIST): Add tests/*.key and tests/ed25519.sec.
</span></span></td><td>Ludovic Courtès</td></tr>