"Building" meaning the OBJECT, and "Building" meaning the PROCESS have separate histories. The "ing" suffixes were originally different.
The origins of the forms:
The NOUN form -- in Old English, the suffix -ung (sometimes -ing) was often used to create nouns. This exact ending is still seen in German nouns that end with -ung. The noun can relate to the original verb ("build") in several ways. Often it refers to something that is the RESULT of the process the verb refers to.
http://en.wiktionary.org/wiki/-ung
http://www.tu-chemnitz.de/phil/english/chairs/linguist/real/independent/eafrica/Diss_Diana/dissch4-3.htm
There are MANY examples of such words in Modern English. Consider the following houns which refer to objects which, like "building", are ordinarily NOT "in process" when we speak of them:
painting, dwelling, saying, writing, gathering, bedding, roofing, sacking, clothing, earnings
(There are other, related ways, that Old English formed nouns from verb roots. One of these is the suffix -(at)ion, such as: decision, isolation, explanation. Another is -ment: establishment.)
The VERB form -- in Old English, participles ended with -ende (a form still found in German).
In Middle English these forms fell together. That is, both the participle and noun ends became "-ing". In fact, the use of this form in expressions like "the building of" is a LATE development (whereas "building" as an object is a very old sort of form).
http://www.everything2.com/index.pl?node_id=261832
For the distinction in the suffixes, see Webster's (1913)
http://www.everything2.com/index.pl?node_id=261832
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Note that it is actually quite common for suffixes that look exactly the same to be used in different parts of a language to carry out different functions. This is not usually a problem and should not confuse speakers because the WAY the forms are used in sentences makes it clear which form is meant. Examples:
-(e)r -- turns a verb into a agentive noun ("maker", "hitter")
OR for the comparative form of an adjective ("bigger")
-(e)s -- a singular present tense verb ending [originally -eth, as in King James "cometh] ("he hits")
OR to make a plural noun form ("hits")
OR (with an apostrophe) to form a possessive
-(e)st -- old ending for verb forms (2nd singular) such as "thou makest"
OR for superlative of adjectives ("finest")
-y -- to form a diminutive ("Timmy")
OR to form adjectives from nouns ("funny")