use pattern rules instead of suffix rules in the Makefile

[letoh]

Both the two suffix rules in current Makefile include prerequisites, so a make with POSIX mode enabled will treat them as rules to build two normal files with strange names (.c.o and .c.eo), rather than create pattern rules implicitly.

According to the release note from make 4.3, the POSIX behavior will be adopted as the only behavior in a future release, so use the pattern rules instead.

As I have mentioned in #53 , the use of suffix rules in current Makefile is incorrect. So the make cannot find any rule to build the target pf_cglue.o. This PR should fix #76 .

References:

• https://lwn.net/Articles/810071/
• https://www.gnu.org/software/make/manual/html_node/Suffix-Rules.html

[philburk]

@letoh - Thanks! I verified that this:

Fixes #52