Add patch to allow PGI to build M4

var/spack/repos/builtin/packages/m4/inline-pgi.patch

@@ -0,0 +1,10 @@
+--- a/m4/extern-inline.m4
++++ b/m4/extern-inline.m4
+@@ -34,6 +34,7 @@
+       ? defined __GNUC_STDC_INLINE__ && __GNUC_STDC_INLINE__ \
+       : (199901L <= __STDC_VERSION__ \
+          && !defined __HP_cc \
++         && !defined __PGI \
+          && !(defined __SUNPRO_C && __STDC__))) \
+      && !defined _GL_EXTERN_INLINE_STDHEADER_BUG)
+ # define _GL_INLINE inline

var/spack/repos/builtin/packages/m4/package.py

@@ -7,6 +7,8 @@ class M4(Package):
 
     version('1.4.17', 'a5e9954b1dae036762f7b13673a2cf76')
 
+    patch('inline-pgi.patch', when='@1.4.17')
+
     variant('sigsegv', default=True, description="Build the libsigsegv dependency")
 
     depends_on('libsigsegv', when='+sigsegv')