diff --git a/ext/fputils/configure.in b/ext/fputils/configure.in
index 59a21c18e..bb2291b6f 100644
--- a/ext/fputils/configure.in
+++ b/ext/fputils/configure.in
@@ -12,10 +12,14 @@ DX_INIT_DOXYGEN([libfputils])
 AC_REQUIRE_AUX_FILE([tap-driver.sh])
 
 AC_PROG_CC
-AM_PROG_CC_C_O
+AC_PROG_CC_C99
 AC_PROG_LIBTOOL
 AC_PROG_AWK
 
+if test "x$ac_cv_prog_cc_c99" = "xno"; then
+  AC_MSG_ERROR([Could not enable C99 support in compiler.])
+fi
+
 AM_CFLAGS="-Wall -Werror"
 AM_CPPFLAGS="-I\$(abs_top_srcdir)/include"
 
diff --git a/ext/fputils/fp80.c b/ext/fputils/fp80.c
index 703b07ac9..8100a4d99 100644
--- a/ext/fputils/fp80.c
+++ b/ext/fputils/fp80.c
@@ -36,10 +36,8 @@
 #include <stdio.h>
 
 typedef union {
-    union {
-        uint64_t bits;
-        double value;
-    };
+    uint64_t bits;
+    double value;
 } fp64_t;
 
 
@@ -74,7 +72,7 @@ build_fp64(int sign, uint64_t frac, int exp)
 int
 fp80_sgn(fp80_t fp80)
 {
-    return (fp80.u.repr.se & FP80_SIGN_BIT) ? -1 : 1;
+    return (fp80.repr.se & FP80_SIGN_BIT) ? -1 : 1;
 }
 
 int
@@ -105,9 +103,9 @@ fp80_isqnan(fp80_t fp80)
 int
 fp80_isqnani(fp80_t fp80)
 {
-    const uint64_t frac_low = fp80.u.repr.fi & (FP80_FRAC_MASK >> 1);
+    const uint64_t frac_low = fp80.repr.fi & (FP80_FRAC_MASK >> 1);
 
-    return fp80_isqnan(fp80) && (fp80.u.repr.se & FP80_SIGN_BIT) && !frac_low;
+    return fp80_isqnan(fp80) && (fp80.repr.se & FP80_SIGN_BIT) && !frac_low;
 }
 
 int
@@ -133,7 +131,7 @@ fp80_isnan(fp80_t fp80)
 int
 fp80_iszero(fp80_t fp80)
 {
-    return fp80.u.repr.fi == 0 && FP80_EXP(fp80) == 0 ? fp80_sgn(fp80) : 0;
+    return fp80.repr.fi == 0 && FP80_EXP(fp80) == 0 ? fp80_sgn(fp80) : 0;
 }
 
 int
@@ -169,10 +167,10 @@ fp80_classify(fp80_t fp80)
 double
 fp80_cvtd(fp80_t fp80)
 {
-    const int sign = fp80.u.repr.se & FP80_SIGN_BIT;
+    const int sign = fp80.repr.se & FP80_SIGN_BIT;
 
     if (!fp80_isspecial(fp80)) {
-        const uint64_t frac = fp80.u.repr.fi;
+        const uint64_t frac = fp80.repr.fi;
         const int unb_exp = FP80_EXP(fp80) - FP80_EXP_BIAS;
         const int fp64_exp = unb_exp + FP64_EXP_BIAS;
         const uint64_t fp64_frac = frac >> (FP80_FRAC_BITS - FP64_FRAC_BITS);
@@ -242,6 +240,6 @@ void
 fp80_debug_dump(FILE *fout, fp80_t fp80)
 {
     fprintf(fout, "sgn: %i, int: %i, frac: 0x%llx, exp: 0x%x (%i)\n",
-            fp80_sgn(fp80), !!(fp80.u.repr.fi & FP80_INT_BIT), FP80_FRAC(fp80),
+            fp80_sgn(fp80), !!(fp80.repr.fi & FP80_INT_BIT), FP80_FRAC(fp80),
             FP80_EXP(fp80), FP80_EXP(fp80) - FP80_EXP_BIAS);
 }
diff --git a/ext/fputils/fpbits.h b/ext/fputils/fpbits.h
index 9fb934843..335a122d3 100644
--- a/ext/fputils/fpbits.h
+++ b/ext/fputils/fpbits.h
@@ -76,15 +76,15 @@
 
 #define BUILD_FP80(sign, frac, exp)                             \
     {                                                           \
-        .u.repr.se = BUILD_FP80_SE(sign, exp),                  \
-        .u.repr.fi = BUILD_FP80_FI(frac, exp)                   \
+        .repr.se = BUILD_FP80_SE(sign, exp),                    \
+        .repr.fi = BUILD_FP80_FI(frac, exp)                     \
     }
 
 #define FP80_FRAC(fp80)                                         \
-    (fp80.u.repr.fi & FP80_FRAC_MASK)
+    (fp80.repr.fi & FP80_FRAC_MASK)
 
 #define FP80_EXP(fp80)                                          \
-    (fp80.u.repr.se & FP80_EXP_MASK)
+    (fp80.repr.se & FP80_EXP_MASK)
 
 #define FP64_FRAC(fp64)                                         \
     (fp64.bits & FP64_FRAC_MASK)
diff --git a/ext/fputils/include/fputils/fp80.h b/ext/fputils/include/fputils/fp80.h
index e584baab4..70acb6cb1 100644
--- a/ext/fputils/include/fputils/fp80.h
+++ b/ext/fputils/include/fputils/fp80.h
@@ -46,14 +46,12 @@ extern "C" {
  */
 
 /** Internal representation of an 80-bit float. */
-typedef struct  {
-    union {
-        char bits[10];
-        struct {
-            uint64_t fi;
-            uint16_t se;
-        } repr;
-    } u;
+typedef union  {
+    char bits[10];
+    struct {
+        uint64_t fi;
+        uint16_t se;
+    } repr;
 } fp80_t;
 
 /** Constant representing +inf */
diff --git a/ext/fputils/tests/fp80_cvtd.c b/ext/fputils/tests/fp80_cvtd.c
index 425a5c44a..5a49d3426 100644
--- a/ext/fputils/tests/fp80_cvtd.c
+++ b/ext/fputils/tests/fp80_cvtd.c
@@ -35,6 +35,15 @@
 #include <stdio.h>
 #include <stdlib.h>
 
+/* We provide our own version of isinf_sgn since the C99 standard
+ * doesn't guarantee that isinf() returns the sign of the infinity
+ * (most implementations do). */
+static inline int
+isinf_sgn(double x)
+{
+    return isinf(x) ? (signbit(x) ? -1 : 1) : 0;
+}
+
 static void
 test_fp80_cvtd_class(const char *name, fp80_t fin, int class)
 {
@@ -48,10 +57,10 @@ test_fp80_cvtd_class(const char *name, fp80_t fin, int class)
 }
 
 static void
-test_fp80_cvtd_inf(const char *name, fp80_t fin, int inf_class)
+test_fp80_cvtd_inf(const char *name, fp80_t fin, int expected_inf_class)
 {
     double d = fp80_cvtd(fin);
-    if (isinf(d) != inf_class) {
+    if (isinf_sgn(d) != expected_inf_class) {
         test_diag("wrong infinity type");
         test_fail(name);
     } else {