It shall not be an unfair labor practice under subsections (a) and (b) of this section for an employer engaged primarily in the building and construction industry to make an agreement covering employees engaged (or who, upon their employment, will be engaged) in the building and construction industry with a labor organization of which building and construction employees are members (not established, maintained, or assisted by any action defined in subsection (a) of this section as an unfair labor practice) because

1. the majority status of such labor organization has not been established under the provisions of section 159 of this title prior to the making of such agreement, or

2. such agreement requires as a condition of employment, membership in such labor organization after the seventh day following the beginning of such employment or the effective date of the agreement, whichever is later, or

3. such agreement requires the employer to notify such labor organization of opportunities for employment with such employer, or gives such labor organization an opportunity to refer qualified applicants for such employment, or

4. such agreement specifies minimum training or experience qualifications for employment or provides for priority in opportunities for employment based upon length of service with such employer, in the industry or in the particular geographical area:

Provided, That nothing in this subsection shall set aside the final proviso to subsection (a)(3) of this section: Provided further, That any agreement which would be invalid, but for clause (1) of this subsection, shall not be a bar to a petition filed pursuant to section 159(c) or 159(e) of this title.