Fix build on MIPS of linux

MIPS of Linux platform is supported frexpl(3).
This fixes to use the frexpl that are provided with gcc if user wants to build
on MIPS of Linux platform.
Signe-doff-by: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>

src/fmt_fp.c

@@ -90,7 +90,7 @@ fmt_u(uint32_t x, char *s)
 typedef char compiler_defines_long_double_incorrectly[9-(int)sizeof(long double)];
 #endif
 
-#if defined(__CYGWIN32__) || defined(__NetBSD__) || defined(mips)
+#if (defined(__CYGWIN32__) || defined(__NetBSD__) || defined(mips)) && !defined(__linux__)
 static long double
 frexpl (long double x, int *eptr)
 {