MAINT: Style check *.c and *.h files

`cpplint` was introduced pandas-dev#14740, and this commit extends to check
other `*.c` and `*.h` files.    Currently, they all reside in
`pandas/src`, and this commit expands the lint to check all of the
following:    1) `datetime` (dir)  2) `ujson` (dir)  3)
`period_helper.c`  4) `All header files`    The parser directory was
handled in pandas-dev#14740, and the others have been deliberately omitted per
the discussion <a href="https://github.com/pandas-
dev/pandas/pull/14740#issuecomment-265260209">here</a>.

Author: gfyoung <[email protected]>

Closes pandas-dev#14814 from gfyoung/c-style-continue and squashes the following commits:

27d4d46 [gfyoung] MAINT: Style check *.c and *.h files

Author: gfyoung

ci/lint.sh

@@ -38,13 +38,22 @@ if [ "$LINT" ]; then
     # readability/casting: Warnings about C casting instead of C++ casting
     # runtime/int: Warnings about using C number types instead of C++ ones
     # build/include_subdir: Warnings about prefacing included header files with directory
+
+    # We don't lint all C files because we don't want to lint any that are built
+    # from Cython files nor do we want to lint C files that we didn't modify for
+    # this particular codebase (e.g. src/headers, src/klib, src/msgpack). However,
+    # we can lint all header files since they aren't "generated" like C files are.
     pip install cpplint
 
     echo "Linting *.c and *.h"
-    cpplint --extensions=c,h --headers=h --filter=-readability/casting,-runtime/int,-build/include_subdir --recursive pandas/src/parser
-    if [ $? -ne "0" ]; then
-        RET=1
-    fi
+    for path in '*.h' 'period_helper.c' 'datetime' 'parser' 'ujson'
+    do
+        echo "linting -> pandas/src/$path"
+        cpplint --extensions=c,h --headers=h --filter=-readability/casting,-runtime/int,-build/include_subdir --recursive pandas/src/$path
+        if [ $? -ne "0" ]; then
+            RET=1
+        fi
+    done
     echo "Linting *.c and *.h DONE"
 
     echo "Check for invalid testing"