primecount-7.7

This is a bug fix release.

• api.cpp: Fix pi(-1) crash #64. Now pi(-1) returns 0 without reporting any error. (Same behavior for all other negative numbers i.e. pi(-n) = 0)
• test/api.cpp: Add pi(-1) test.
• test/api_c.c: Add primecount_pi(-1) test. Convert api_c.cpp to api_c.c.
• test/nthprime.cpp: Add new test.
• primecount.h: Fix -Wstrict-prototypes warning.