Building on agricultural land?

Yes,You can but you will have to convert the agricultural land to non-agricultural purposes,
Procedure For Application For Conversion Of Agricultural Land
The application for conversion should be made in the name of the "Owner" and duly signed by the person entitled to do so. The application form needs to be submitted to the Ministry together with the following all important documentation:
9 copies of title deed
Certified and precise site and location plan drawn by a sworn land surveyor (21 photocopies) clearly indicating identifiable landmarks
Photocopy of National Identity Card of applicant
Photocopy of affidavit of succession, if applicable, or any other documents as may be required by the Ministry.

All the documents are required to sent by Registered post to the address mentioned here in after:

Permanent Secretary
Ministry of Agro-Industry and Food
9th Floor Renganaden Seeneevassen Building
Port Louis.
Tel No.: 212-0854, 212-2940 (Ext 2015)

source: http://www.realestatemauritius.mu/en/land-in-mauritius/useful-information/agricultural-land-in-mauritius.html