Work around broken floating point limits on some systems.

The values of {FLT,DBL}_{MAX,MIN} macros on some systems (older musl
libc, some BSD flavours) are not exactly representable, i.e.
(double)DBL_MAX == DBL_MAX is false
This violates (at least some interpretations of) the C99 standard and
breaks code (e.g. in vf_fps) like
double f = DBL_MAX;
[...]
if (f == DBL_MAX) { // f has not been changed yet
    [....]
}

compat/float/float.h

+/*
+ * Work around broken floating point limits on some systems.
+ *
+ * This file is part of Libav.
+ *
+ * Libav is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * Libav is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with Libav; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+#include_next <float.h>
+
+#ifdef FLT_MAX
+#undef  FLT_MAX
+#define FLT_MAX 3.40282346638528859812e+38F
+
+#undef  FLT_MIN
+#define FLT_MIN 1.17549435082228750797e-38F
+
+#undef  DBL_MAX
+#define DBL_MAX ((double)1.79769313486231570815e+308L)
+
+#undef  DBL_MIN
+#define DBL_MIN ((double)2.22507385850720138309e-308L)
+#endif

compat/float/limits.h

+/*
+ * Work around broken floating point limits on some systems.
+ *
+ * This file is part of Libav.
+ *
+ * Libav is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * Libav is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with Libav; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+#include_next <limits.h>
+#include <float.h>

configure

@@ -994,6 +994,16 @@ check_builtin(){
     check_code ld "$headers" "$builtin" "$@" && enable "$name"
 }
 
+check_compile_assert(){
+    log check_compile_assert "$@"
+    name=$1
+    headers=$2
+    condition=$3
+    shift 3
+    disable "$name"
+    check_code cc "$headers" "char c[2 * !!($condition) - 1]" "$@" && enable "$name"
+}
+
 require(){
     name="$1"
     header="$2"
@@ -1360,6 +1370,7 @@ HAVE_LIST="
     fast_clz
     fast_cmov
     fcntl
+    flt_lim
     fork
     getaddrinfo
     gethrtime
@@ -3536,6 +3547,9 @@ if enabled_all ccc libc_glibc; then
     add_ldflags -Wl,-z,now  # calls to libots crash without this
 fi
 
+check_compile_assert flt_lim "float.h limits.h" "DBL_MAX == (double)DBL_MAX" ||
+    add_cppflags '-I\$(SRC_PATH)/compat/float'
+
 esc(){
     echo "$*" | sed 's/%/%25/g;s/:/%3a/g'
 }