win32/build.py: add -march=pentium3 to fix 32 bit LAME build

Workaround for the following LAME build failure:

 error: inlining failed in call to always_inline '_mm_sqrt_ps': target
 specific option mismatch

This is because the LAME build scripts do not check whether SSE is
available; they only check for the presence of the "xmmintrin.h"
header.

Requiring a Pentium 3 CPU is reasonable enough, and it's the first CPU
to feature SSE support.

win32/build.py

@@ -51,6 +51,11 @@ class CrossGccToolchain:
         self.strip = os.path.join(toolchain_bin, arch + '-strip')
 
         common_flags = ''
+
+        if not x64:
+            # enable SSE support which is required for LAME
+            common_flags += ' -march=pentium3'
+
         self.cflags = '-O2 -g ' + common_flags
         self.cxxflags = '-O2 -g ' + common_flags
         self.cppflags = '-isystem ' + os.path.join(install_prefix, 'include')