MSVS : Define _USE_MATH_DEFINES.

MSVS does defined the M_LN2 constant in <math.h> but only makes it
visible if _USE_MATH_DEFINES is defined.

Patch-from: lvqcl <lvqcl.mail@gmail.com>

src/libFLAC/fixed.c

@@ -42,7 +42,6 @@
 #include "FLAC/assert.h"
 
 #ifndef M_LN2
-/* math.h in VC++ doesn't seem to have this (how Microsoft is that?) */
 #define M_LN2 0.69314718055994530942
 #endif
 

src/libFLAC/libFLAC_dynamic.vcproj

@@ -38,6 +38,7 @@
 			/>
 			<Tool
 				Name="VCCLCompilerTool"
+				AdditionalOptions="/D &quot;_USE_MATH_DEFINES&quot;"
 				Optimization="0"
 				AdditionalIncludeDirectories=".\include;..\..\include"
 				PreprocessorDefinitions="WIN32;_DEBUG;_WINDOWS;_USRDLL;FLAC_API_EXPORTS;FLAC__HAS_OGG;FLAC__CPU_IA32;FLAC__HAS_NASM;FLAC__USE_3DNOW;FLAC__ALIGN_MALLOC_DATA;VERSION=\&quot;1.3.0\&quot;;DEBUG;FLAC__OVERFLOW_DETECT"
@@ -114,6 +115,7 @@
 			/>
 			<Tool
 				Name="VCCLCompilerTool"
+				AdditionalOptions="/D &quot;_USE_MATH_DEFINES&quot;"
 				EnableIntrinsicFunctions="true"
 				FavorSizeOrSpeed="1"
 				OmitFramePointers="true"

src/libFLAC/libFLAC_static.vcproj

@@ -38,6 +38,7 @@
 			/>
 			<Tool
 				Name="VCCLCompilerTool"
+				AdditionalOptions="/D &quot;_USE_MATH_DEFINES&quot;"
 				Optimization="0"
 				AdditionalIncludeDirectories=".\include;..\..\include"
 				PreprocessorDefinitions="WIN32;_DEBUG;_LIB;FLAC__HAS_OGG;FLAC__CPU_IA32;FLAC__HAS_NASM;FLAC__USE_3DNOW;FLAC__ALIGN_MALLOC_DATA;VERSION=\&quot;1.3.0\&quot;;FLAC__NO_DLL;DEBUG;FLAC__OVERFLOW_DETECT"
@@ -103,6 +104,7 @@
 			/>
 			<Tool
 				Name="VCCLCompilerTool"
+				AdditionalOptions="/D &quot;_USE_MATH_DEFINES&quot;"
 				EnableIntrinsicFunctions="true"
 				FavorSizeOrSpeed="1"
 				OmitFramePointers="true"

src/libFLAC/lpc.c

@@ -52,7 +52,6 @@
 #ifndef FLAC__INTEGER_ONLY_LIBRARY
 
 #ifndef M_LN2
-/* math.h in VC++ doesn't seem to have this (how Microsoft is that?) */
 #define M_LN2 0.69314718055994530942
 #endif