wscript: add LIBRARY_PATH for library detection

MinGW GCC seems to ignore LIBRARY_PATH which causes problem
when some libraries not using pkg-config were installed to
local directory

waftools/checks/custom.py

@@ -50,6 +50,8 @@ def check_iconv(ctx, dependency_identifier):
     if ctx.env.DEST_OS == 'openbsd' or ctx.env.DEST_OS == 'freebsd':
         args['cflags'] = '-I/usr/local/include'
         args['linkflags'] = '-L/usr/local/lib'
+    elif ctx.env.DEST_OS == 'win32':
+        args['linkflags'] = " ".join(['-L' + x for x in ctx.env.LIBRARY_PATH])
     checkfn = check_cc(**args)
     return check_libs(libs, checkfn)(ctx, dependency_identifier)
 

wscript

@@ -989,6 +989,8 @@ def configure(ctx):
     ctx.find_program(windres,     var='WINDRES',   mandatory=False)
     ctx.find_program('perl',      var='BIN_PERL',  mandatory=False)
 
+    ctx.add_os_flags('LIBRARY_PATH')
+
     ctx.load('compiler_c')
     ctx.load('waf_customizations')
     ctx.load('dependencies')