iss: make mfc*.dll and msvc*.dll optional

mfc71.dll was only needed for Python2.4
msvc*.dll is similarly optional
Do not bail installer build if not found

contrib/win32/mercurial.iss

@@ -54,8 +54,8 b' Source: contrib\\win32\\postinstall.txt; D'
 Source: dist\hg.exe; DestDir: {app}; AfterInstall: Touch('{app}\hg.exe.local')
 Source: dist\python*.dll; Destdir: {app}; Flags: skipifsourcedoesntexist
 Source: dist\library.zip; DestDir: {app}
-Source: dist\mfc*.dll; DestDir: {app}
-Source: dist\msvc*.dll; DestDir: {app}
+Source: dist\mfc*.dll; DestDir: {app}; Flags: skipifsourcedoesntexist
+Source: dist\msvc*.dll; DestDir: {app}; Flags: skipifsourcedoesntexist
 Source: dist\Microsoft.VC*.CRT.manifest; DestDir: {app}; Flags: skipifsourcedoesntexist
 Source: dist\Microsoft.VC*.MFC.manifest; DestDir: {app}; Flags: skipifsourcedoesntexist
 Source: dist\w9xpopen.exe; DestDir: {app}