build

Ignore auto-generated extconf.h in lib directory #70

Job	Run time
Build on ruby-head-debug (debug)	1m 58s
fedora	2m 45s
ubuntu ruby-3.3	1m 2s
ubuntu ruby-2.7	1m 22s
ubuntu ruby-3.1	1m 1s
ubuntu ruby-3.0	1m 1s
ubuntu ruby-2.6	1m 26s
macos ruby-3.0	38s
macos ruby-head	33s
macos ruby-2.6	1m 11s
ubuntu ruby-3.2	1m 5s
ubuntu ruby-head	1m 15s
windows ruby-3.0	2m 27s
ubuntu ruby-3.4	1m 11s
macos ruby-3.3	33s
windows ruby-3.1	2m 34s
macos ruby-3.1	46s
windows ruby-2.6	3m 19s
macos ruby-3.2	31s
windows ruby-head	2m 24s
windows ruby-2.7	3m 11s
windows ruby-3.4	2m 14s
windows ruby-3.3	2m 29s
macos ruby-2.7	59s
macos ruby-3.4	34s
windows ruby-3.2	2m 30s
	40m 59s

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