diff --git a/liba/Makefile b/liba/Makefile
index 36778c01b..074eff25e 100644
--- a/liba/Makefile
+++ b/liba/Makefile
@@ -99,7 +99,7 @@ objs += $(addprefix liba/src/external/openbsd/, \
   w_lgamma.o \
 )
 
-liba/src/external/openbsd/%.o: CFLAGS += -Iliba/src/external/openbsd/include
+liba/src/external/openbsd/%.o: SFLAGS := -Iliba/src/external/openbsd/include $(SFLAGS)
 liba/src/external/openbsd/e_lgammaf_r.o: CFLAGS += -w
 liba/src/external/openbsd/s_log1pf.o: CFLAGS += -w
 liba/src/external/openbsd/s_scalbnf.o: CFLAGS += -w
diff --git a/liba/include/math.h b/liba/include/math.h
index 95cd641fa..6d20e74e6 100644
--- a/liba/include/math.h
+++ b/liba/include/math.h
@@ -73,6 +73,39 @@ float sqrtf(float x);
 float tanf(float x);
 float tanhf(float x);
 
+double acos(double x);
+double acosh(double x);
+double asin(double x);
+double asinh(double x);
+double atan(double x);
+double atanh(double x);
+double ceil(double x);
+double copysign(double x, double y);
+double cos(double x);
+double cosh(double x);
+double exp(double x);
+double expm1(double x);
+double fabs(double x);
+double floor(double x);
+double lgamma(double x);
+double lgamma_r(double x, int *signgamp);
+double log1p(double x);
+double log10(double x);
+double log(double x);
+double pow(double x, double y);
+double round(double x);
+double scalbn(double x, int n);
+double sin(double x);
+double sinh(double x);
+double sqrt(double x);
+double tan(double x);
+double tanh(double x);
+
+/* The C99 standard says that any libc function can be re-declared as a macro.
+ * (See N1124 paragraph 7.1.4). This means that C files willing to actually
+ * implement said functions should either re-define the prototype or #undef the
+ * macro. */
+
 #define acosf(x) __builtin_acosf(x)
 #define acoshf(x) __builtin_acoshf(x)
 #define asinf(x) __builtin_asinf(x)
@@ -105,34 +138,6 @@ float tanhf(float x);
 #define tanf(x) __builtin_tanf(x)
 #define tanhf(x) __builtin_tanhf(x)
 
-double acos(double x);
-double acosh(double x);
-double asin(double x);
-double asinh(double x);
-double atan(double x);
-double atanh(double x);
-double ceil(double x);
-double copysign(double x, double y);
-double cos(double x);
-double cosh(double x);
-double exp(double x);
-double expm1(double x);
-double fabs(double x);
-double floor(double x);
-double lgamma(double x);
-double lgamma_r(double x, int *signgamp);
-double log1p(double x);
-double log10(double x);
-double log(double x);
-double pow(double x, double y);
-double round(double x);
-double scalbn(double x, int n);
-double sin(double x);
-double sinh(double x);
-double sqrt(double x);
-double tan(double x);
-double tanh(double x);
-
 #define acos(x) __builtin_acos(x)
 #define acosh(x) __builtin_acosh(x)
 #define asin(x) __builtin_asin(x)
diff --git a/liba/src/external/openbsd/include/math.h b/liba/src/external/openbsd/include/math.h
new file mode 100644
index 000000000..f3457a210
--- /dev/null
+++ b/liba/src/external/openbsd/include/math.h
@@ -0,0 +1,66 @@
+#include_next <math.h>
+
+/* In accordance with the C99 standard, we've defined libm function as macros to
+ * leverage compiler optimizations. When comes the time to actually implement
+ * those functions, we don't want the macro to be active. OpenBSD doesn't use
+ * macros so it doesn't bother #undef-ing libm functions. Let's do it here. */
+
+#undef acosf
+#undef acoshf
+#undef asinf
+#undef asinhf
+#undef atanf
+#undef atan2f
+#undef atanhf
+#undef ceilf
+#undef copysignf
+#undef cosf
+#undef coshf
+#undef expf
+#undef expm1f
+#undef fabsf
+#undef floorf
+#undef fmodf
+#undef lgammaf
+#undef lgammaf_r
+#undef log1pf
+#undef log10f
+#undef logf
+#undef nanf
+#undef nearbyintf
+#undef powf
+#undef roundf
+#undef scalbnf
+#undef sinf
+#undef sinhf
+#undef sqrtf
+#undef tanf
+#undef tanhf
+
+#undef acos
+#undef acosh
+#undef asin
+#undef asinh
+#undef atan
+#undef atanh
+#undef ceil
+#undef copysign
+#undef cos
+#undef cosh
+#undef exp
+#undef expm1
+#undef fabs
+#undef floor
+#undef lgamma
+#undef lgamma_r
+#undef log1p
+#undef log10
+#undef log
+#undef pow
+#undef round
+#undef scalbn
+#undef sin
+#undef sinh
+#undef sqrt
+#undef tan
+#undef tanh
diff --git a/liba/src/nearbyintf.c b/liba/src/nearbyintf.c
index ca0e261e6..f62e02fb8 100644
--- a/liba/src/nearbyintf.c
+++ b/liba/src/nearbyintf.c
@@ -1,5 +1,7 @@
 #include <math.h>
 
+#undef nearbyintf
+
 /* nearbyintf is not supposed to be the same as roundf according to openBSD
  * documentation. Indeed:
  * - they round halfway cases to the nearest integer instead of away from zero