libiconv,lmod: improvements for static builds (#29493)

* libiconv: compile with pic even when static build

* lmod: require shared lua

It seems to be unable to detect lua-posix when using a static lua:
```
Error: The follow lua module(s) are missing:  posix
```

var/spack/repos/builtin/packages/libiconv/package.py

@@ -31,6 +31,7 @@ def configure_args(self):
         args = ['--enable-extra-encodings']
 
         args += self.enable_or_disable('libs')
+        args.append('--with-pic')
 
         # A hack to patch config.guess in the libcharset sub directory
         copy('./build-aux/config.guess',

var/spack/repos/builtin/packages/lmod/package.py

@@ -45,7 +45,7 @@ class Lmod(AutotoolsPackage):
     version('6.4.1',  sha256='a260b4e42269a80b517c066ba8484658362ea095e80767a2376bbe33d9b070a5')
     version('6.3.7',  sha256='55ddb52cbdc0e2e389b3405229336df9aabfa582c874f5df2559ea264e2ee4ae')
 
-    depends_on('lua@5.1:')
+    depends_on('lua+shared@5.1:')
     depends_on('lua-luaposix', type=('build', 'run'))
     depends_on('lua-luafilesystem', type=('build', 'run'))
     depends_on('tcl', type=('build', 'link', 'run'))